The search for the British crew of the yacht Cheeki Rafiki will be suspended if nothing is found by tonight (Friday), US coastguards have said.
In a statement they said the search would continue through the night and into today.
But if nothing was found of Hampshire sailor James Male and his fellow crew members then the serach will be called off by midnight.
US Coastguard also said that the families of the missing crew had been told of the decision.
Captain Anthony Popiel, pictured below, said he had spoken to the family of the four missing sailors to tell them that the search would be suspended at midnight.
He said: ''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.
Capt Popiel said: ''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.''
Capt Popiel said: ''Unfortunately we have had no sightings thus far and have concluded that none of the debris or objects located during the search have correlated to the Cheeki Rafiki.''
More rescue vessels would arrive at the scene today, he said.
The coastguard cutter Vigorous will arrive on scene within the next few hours and a US navy warship has already arrived in the search area and is using its helicopter to carry out searches.
Capt Popiel said: ''In total, efforts since resuming the search have covered more than 17,500 square miles of ocean.''
Foreign Office Minister Hugh Robertson also spoke with the families of the missing sailors last night to inform them of the US Coast Guard's decision to suspend the search, saying it had gone ''above and beyond'' in its efforts to find them.
Mr Robertson said: ''I know that, despite there being no further sightings of the Cheeki Raffiki or its crew, any decision to suspend the search will be incredibly difficult and will only be taken after the most serious deliberation.''
Search conditions yesterday were described as ''favourable'', with winds around 20 knots and seas of roughly 6ft to 8ft.
It comes after relatives of James Male told that the search is being boosted by extra air support.
James Male's father and sister were back at the Foreign Office this afternoon to hear the lateast on the search efforts.
They were told that rescue crews desperately hunting for the 23-year-old from Romsey and his three crew mates in the Atlantic have yet to spot signs of their life raft - three days after resuming their search.
But relatives of the Cheeki Rafiki crew were told this afternoon that the operation - which has so far scoured 12,000 square miles of ocean - is being boosted by additional air support and seven aircraft.
And now relatives of the men said the operation was being boosted by additional air support and seven aircraft.
This includes three US military aircraft will join the search, alongside two Canadian planes, one from Britain and one from the US Coast Guard.
After today's meeting James' father Graham said: “It was a very constructive meeting, it really filled the families in with the detail.
''What we can say is the UK Government and the US Coast Guard are right behind us, which we're so grateful for.
"We know there's going to be some resources going out there. Of course it's a large area to cover. They are going to continue their support.''
Cressida Goslin, wife of Paul, added: ''We're very pleased, it's wonderful news. I don't think anybody could be doing anything more than they are.''
Hugh Robertson, minister for North America, said: ''The UK Government remains in constant contact with our US colleagues and I was able to update them on the continuing search.
''I'd like to thank the US Coastguard for their significant efforts thus far, and to assure everybody that the UK Government will continue to do everything possible to try and locate these missing yachtsmen.''
The sailors' families are also 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 disappearance of 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 co-ordinates to the coastguard or the yachts that are out there.''
The 40-ft vessel ran into difficulties when ot started taking on water 620 miles east of Cape Cod, Massachusetts, last Thursday.
While sailing back to Britain from an Antigua regatta.
The four men on board - experienced captain Andrew Bridge, 22, from Farnham in Surrey, and crew members James Male, Steve Warren, 52, from Bridgwater, Somerset, and Paul Goslin, 56, from West Camel, Somerset - have not been seen or heard from since the early hours of Friday morning.
The US Coastguard initially called off the search for the missing men on Sunday after spending two days hunting.
But they recommenced their efforts after pressure from the UK Government, families and an online petition which reached 233,142 signatures yesterday.