Three crewmen died after they were rescued from a capsized vessel off Shetland and it will be "a miracle" if five others are found alive, a Maritime and Coastguard Agency spokeswoman said today.
The five who are missing are believed to include a boy aged 15 and his father.
Navy divers equipped with heat seeking cameras are still searching for the five missing crewmen although darkness and strong currents prevented them from carrying out a planned search under the vessel. They hope to try again this morning.
Rescue helicopters which searched the area last night had been expected to go back out over the North Sea at first light today, but a spokeswoman for the coastguard said they were no longer being sent out.
"We think the crew are still within the vicinity of the vessel which is still bobbing about on the surface. It hasn't sunk," she said.
The spokeswoman added it was thought that the helicopters would not be able to see the men, who are believed to be trapped in the hull, so the decision was taken early today to postpone a second helicopter search.
She admitted it was unlikely that the five missing men would be found alive.
"We are not really expecting to find survivors now," she said.
"It would be a miracle if they had survived all night in five degrees, freezing water.
"It's very sad.
"There is a chance there is an air pocket somewhere, but that chance is ever so slim.
"We have to be realistic about it."
She said it is unlikely the men were wearing safety equipment as they were inside the boat at the time it capsized.
The men were part of a 15-strong Norwegian crew aboard the handling supply tug Bourbon Dolphin which overturned at around 5.20pm yesterday while undertaking an operation at the drilling rig Transocean Rather.
Ten of the crew were rescued from the tug which was 75 nautical miles north west of Shetland. Three crewmen were later confirmed as dead.
The other seven rescued crew were airlifted to Gilbert Bain Hospital in Lerwick, Shetland.
The coastguard spokeswoman said all non-essential personnel were taken off the Transocean Rather platform last night as there was a very minimal risk that the capsized vessel could damage the platform.
The ship, an anchor handling vessel, is less than a year old and is owned by Bourbon Norway.
The navy divers were flown to the stricken boat from Faslane on the Clyde last night.
The 250ft ship, which is less than one year old, had been working in the vicinity of the Rosebank oilfield when it capsized just over a mile from the rig.
The immediate cause of the incident was unclear as the ship was performing a routine anchor-handling operation in fair weather conditions.
Aberdeen Coastguard said the chances of any more survivors being found were "extremely slim".
Michael Coull, duty watch manager, told BBC Radio Scotland: "I'm aware of similar incidents happening in other parts of the world. However, it is extremely rare for something like this to happen in this part of the world.
"Weather conditions are not the best that they could be but they're not the worst for the North Atlantic at this time of year
"The visibility is not that good but it's certainly not hampering the operation at the moment.
"The vessel is upside down, which makes it considerably more difficult to find out what's going on inside, if anything.
"The fact that the vessel is still afloat would suggest that there's a possibility that there's still some air pockets inside the vessel.
"And if the remainder of the crew were in a water-tight compartment, then there's a possibility that they could still be alive.
"However, the chances are extremely slim and it's highly unlikely that that's the case."
The 99 crew members of the Transocean Rather rig have all been accounted for and all non-essential personnel transferred off the facility.
Michael Mulford, from the rescue centre at RAF Kinloss, said 72 people have been evacuated from the rig.
He told BBC Radio 4's Today programme he was being "realistic" about the fate of the five missing crewmen.
Mr Mulford said: "I don't want to raise people's hopes unnecessarily, but there is still a chance and where rescue teams see even the smallest chance, it's always worth a calculated risk."
On the possible cause of the accident, Mr Mulford said: "It's probably connected with anchors and things like that, but may be rather technical.
"Whatever happened, happened in a split second."
Grampian Police said it has mobilised its emergency plan and is liaising closely with everyone involved.
Incident commander Superintendent Bill Archibald said: "Our thoughts are with the families of those involved in this traumatic incident."
The Bourbon Dolphin had a number of roles in the North Sea, including anchor handling and towing, the installation of subsea construction blocks and operations involving remote vehicles.
Trond Myklebust, managing director of Bourbon Offshore, which is based in Fosnavaag in the Vestlandet region of Norway, said he had chartered a plane which was due to bring company representatives and family members to Lerwick this afternoon.
He said: "This is the worst day in the company. It's difficult and is tragic for us but we are trying to focus on helping the relatives.
"It is tragic and we are doing everything to assist the families and the teams out there just now.
"At this stage we do not know how it happened. We only know it happened quickly."
He added that there was no mayday or distress call from the ship.
"We have been taking care of relatives here in our office and we have had doctors here and police and we are doing everything we can to assist," he said.
He described his company as a small operator in the industry with 500 seamen and 27 office-based staff."Reuse content