The search for the British crew of the yacht Cheeki Rafiki will be suspended if nothing is found by tonight, the US Coast Guard has said.
Captain Anthony Popiel said he had spoken to the family of the four missing sailors to tell them that the search would be called off at midnight.
It comes after the U.S Coast Guard announced that debris spotted in the Atlantic Ocean is not from the missing yacht.
Captain Popiel said last night: 'I informed them that the search would continue throughout the night and into tomorrow. If by midnight tomorrow (5am Saturday UK time) there are no further developments to indicate search efforts would locate the crew alive we will suspend the search.''It is only after deepest consideration that we suspend active search efforts.'
He added: 'With sincere compassion for the families of these four men, our thoughts and prayers are with them all during this difficult time.'
Coastguard officials have informed the British Consulate of their plans but are continuing to search for the missing men - experienced captain Andrew Bridge, 22, from Farnham in Surrey, and crew members James Male, 23, from Southampton, Steve Warren, 52, from Bridgwater, Somerset, and Paul Goslin, 56, from West Camel, Somerset.
Capt Popiel said: 'Our focus right now however remains with this very active and very dynamic search. We will always put forth utmost efforts to find and rescue those in peril at sea.'
Earlier the crew of an Austrian catamaran named the Malisi spotted a plank of wood - possibly a floorboard or part of a table - and a plastic board in the northern sector of the 130-square-mile search area.
But the U.S Coast Guard said in a statement that the debris was not from the Cheeki Rafiki.
It said: 'A volunteer vessel in the search area reported objects in the water, but they did not correlate to the Cheeki Rafiki search.'
Captain Anthony Popiel, who is overseeing the search, had said earlier: 'During searches at sea, it is not uncommon to find debris or discarded objects. Locating smaller items in the ocean is actually an indicator of favorable seas and search conditions.
'Our search assets have found a variety of debris and trash during their searches. The key part is correlating these objects to the search effort. We take reports of debris very seriously and, at this time, no debris or objects reported during this search correlate to the Cheeki Rafiki.
'This is still a very remote region of the Atlantic and our search planning is focused on directing the U.S. and international military ships and aircraft we have deployed. We are also working directly with volunteer commercial ships in the area. We want to reiterate to all private mariners the inherent risks of operating in that environment.'
The Malisi skipper, Frenchman Patrick Michel, told the BBC earlier: 'We are currently just in the north part of the search area, our third night out here, and we did see during this night a few little [pieces of] debris which I have reported to the US Coast Guard with the times and positions, so there is a little bit of light at the end of the tunnel.'
He said the debris appeared new as it was free of barnacles.
The news comes after a former commandant of the U.S Coast Guard said that the four British sailors are already dead.
Admiral James Loy, head of the U.S Coast Guard between 1998 and 2002, told the Daily Telegraph: 'It would seem to me that hope is pretty much lost at this point.'
Commenting on the decision by the U.S Coast Guard to resume the search, he added that it was probably being done out of respect to the country's closest ally.
On Wednesday an RAF plane joined the multinational search effort.
A C130 Hercules aircraft took off from RAF Brize Norton at 5am on its way to Lajes in the Azores, where it refuelled before flying to the search area over the Atlantic, the Ministry of Defence (MoD) said.
The plane joined a flotilla of small sailing boats and the U.S Coast Guard, which bowed to mounting public and diplomatic pressure from the UK to resume efforts to find the crew of the 40ft Cheeki Rafiki.
Captain Popiel said rescuers have scoured more than 9,000 square miles (23,000 sq kilometers) of ocean and completed at least eight searches since resuming efforts Tuesday morning.
Evoking the spirit of the 1940 evacuation of Dunkirk by a fleet of ‘little ships’, about 40 private yachts are heading to the area where the stricken boat is thought to have drifted since it capsized on Friday.
The boats joining the search include a group of eight yachts and a further 32 vessels from Antigua, many of which have been competing in regattas across the Caribbean.
The skippers of a further 35 yachts taking part in a rally organised by the World Cruising Club have also been asked for help.
On Thursday the number of U.S aircraft involved in the search was boosted by three to five, with a Canadian military plane also scouring an area about 1,000 miles east of Cape Cod in Massachusetts.
There are six ships involved in the operation, too.
Defence Secretary Philip Hammond said: 'The RAF's contribution to the search operation for the four missing British sailors will provide additional capability and resilience to the resumed search led by US and Canadian forces.
'We all hope that the extensive resources being provided by our allies and the further support from the UK can help locate the missing yachtsmen as soon as possible.'
It's possible to pinpoint the location of some of the ships involved in the search using marinetraffic.com, which carries information on the whereabouts of thousands of vessels around the world.
Cargo vessel AM Hamburg is part of the operation and can be seen heading in a northerly direction around 1000 miles east of Cape Cod.
Cheeki Rafiki captain Andrew Bridge, 22, and crew members radioed on Thursday that their yacht was taking on water.
Their families are clinging to hopes that the men managed to get into the boat’s life-raft and are simply awaiting rescue.
Relatives of all four sailors went to the Foreign Office today to be updated on the search.
Gloria Hamlet, Mr Warren's partner, said: ‘If there's eyes out there looking for them then there's a chance.
‘Hopefully today will be the day but we've got to wait and see.
‘At the end of the day we're trying to be very grounded and trying to be very realistic, but you can't help but hope.’
The sailors' families said they are in discussions with a crowd-sourcing initiative set up by a satellite company.
Tomnod, run by DigitalGlobe, allows people to view satellite photos online and tag objects of interest. It was previously used following Typhoon Haiyan and the missing Malaysian Airlines flight MH370.
Mr Warren's daughter, Laura Carpenter, said: 'It's a free website so you just sign up and then you can search images and you can tag if you see anything, then they send the coordinates to the coastguard or the yachts that are out there.'
She said she hoped the system would be set up soon but acknowledged that it was a lengthy procedure.
Mr Warren’s sister Kay Coombes, 46, from Wincanton, Somerset, said: ‘We can only hold out hope they will find something.
'They said they are going to keep their eyes peeled for anything that may help us, so we are clinging on to that at the moment.’
James Male's mother, Lorraine, told the BBC: 'Waiting is horrible, but we'll hear soon. We know that much.'
James' father, Graham, said: 'This search has to work - and is going to find them.
The Male family have set up a family 'ops room', with friends and relatives manning laptops and smartphones around the dining room in their Romsey home to scour the internet for information and to process all the social media attention the story is getting.
There are estimated to be 100 or more yachts sailing across the Atlantic to Europe after spending the winter in the Caribbean.
Their captains have been asked to consider switching course to pass through the search zone – although some will be too far away, hampered by weather or lack of fuel.
Jeremy Wyatt, of the Isle of Wight-based World Cruising Club, said: ‘They are looking for anything – debris, the life- raft. But with every hour that passes, the zone gets bigger.’
He added: ‘The decision whether to divert course to assist is down to individual skippers. We can only make an appeal.’
The US Coast Guard came under fire for ending its search after only two days, although it claimed that the crew could only have survived in the atrocious weather conditions for 20 hours.
Relatives launched a campaign to have the operation resumed, winning the support of celebrities, top sailors, politicians and more than 200,000 people who signed an online petition.
The American authorities finally performed a U-turn in response to requests from the British Government, and sent an aircraft back to the area where the men vanished.
Mr Goslin’s wife Cressida, 51, from West Camel, Somerset, said it has been a ‘complete emotional rollercoaster’ as discussions swung back and forth between the Foreign Office and the US.
Mr Bridge’s grandmother Valerie, 75, added: ‘We are delighted.
'All we wanted was another search. It might not come to anything, but people want them to do it and they are trying.
'It seemed too quick, just two days, and we were saying, “If only they could do it for a bit longer”. You never know what could happen.’
Following ‘intensive discussions’ between ministers and the US authorities, David Cameron wrote on Twitter: ‘My thanks to the US Coastguard, which has resumed its search for our missing yachtsmen.'
The US coastguard, Canadian aircraft and three merchant vessels searched for the sailors throughout Friday and Saturday, with some 4,000 square miles previously scanned for the vessel's two personal-location GPS beacons until no more transmissions were received from the small devices, which have a short battery life.
On Saturday, a cargo vessel which was helping with the search spotted and photographed an overturned hull which matched the description of the Cheeki Rafiki but reported no signs of people on board or a liferaft.
The search was called off on Sunday.
The new search has been using the location of the capsized boat found by a cargo vessel as its starting point.
Earlier, Malisi skipper Mr Michel revealed he is following dolphins in a desperate hunt to find them.
Mr Michel turned his 64 foot yacht around to join the mass mission in the Atlantic ocean after hearing US coastguards were relaunching the hunt.
Mr Michel revealed he saw dolphins as he headed back towards the search area and so changed his route again to follow the way they were pointing.
Frenchman Mr Michel said: 'I have two messages for the men's families.
'Firstly, my gut feeling is telling me this incident will end up in a positive way.
'The second one is more interesting and will make everybody smile. It was the first day yesterday that we saw dolphins.
'They seemed to indicate a certain direction to turn the boat, so that is what we did.
'We started going south instead of going north, which had been my intention.
'This sign is a very positive note from the mammals in the sea.
'We think and we hope the dolphins were helping us. In certain circumstances this has saved ships from coming ashore on rocks and reefs.'
He added: 'A man can stay alive for three minutes without air, three days without water - and three weeks without food so this crew can stay quite a while and we will stay around to find them.'