Four men and a boy are missing, presumed dead, after a boat capsized in the Atlantic claiming the lives of three others.
Seven people survived when the Norwegian anchor handling tug overturned on Thursday.
A coastguard spokesman said it was now "extremely unlikely" that they would be found alive.
Royal Navy divers entered the submerged vessel three times on Friday but found no signs of life.
They were later forced to withdraw amid rough seas.
Seven of the 15 Bourbon Dolphin crew are in hospital while three have been confirmed dead.
Among those still unaccounted for is a 15-year-old boy on work experience and his father, believed to be the ship's captain.
Neville Davis, of Shetland Coastguard, said: "After an intensive search we must now accept that despite tremendous efforts from all rescue units involved it is extremely unlikely that the five missing crew will be found alive and our sympathies are with the families of the crew at this time.
"We would like to thank all the rescue units and vessels involved in this operation who have made every effort possible and thankfully did bring seven survivors to safety."
The operation around the upturned Bourbon Dolphin has now switched focus to the salvage of the vessel, which currently remains afloat.
Ten crew members in total were found in the seas around the vessel after Thursday's incident, about 86 miles west of the Shetland coast.
The mother of one survivor described how he jumped from the boat as it began to flip over.
Turge Drage said her son, Egil Hafsas, had called her from Lerwick to say how he and two other crew members managed to pull on life jackets and leap into the cold sea before being picked up by another ship and airlifted to Shetland.
Families of the crewmen, all Norwegian, were expected to arrive in Shetland later.
They had stayed overnight in the firm's offices where they were counselled by a priest.
A service was held for them on Friday morning before they began the journey to Scotland.
Norwegian-registered tug supply vessel
Max draught: 6.5m
Deadweight (max): 2,500 t
Gross/net tonnage: 2,974t/892t
Speed: 17.5 knots (trial)
Capacity: 35 personnel
Source: Bourbon Offshore
An offshore union leader has said he believed the boat, which is less than one year old, could have been caused to capsize by an anchor slipping up the side of the vessel.
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.
Anchor handling tugs are also used to supply oil platforms, tow them to location, anchor them up and, in some cases, serve as recovery and rescue vessels.
They are fitted with winches for towing and anchor handling and have an open stern to allow the decking of anchors.
The coastguard confirmed that the ship had still been attached to Transocean Rather rig by an anchor chain.
Grampian Police said they would begin an investigation into the incident. A marine accident inquiry will also be carried out.
The ship had been working in the vicinity of the Rosebank oilfield.
Trond Myklebust, manager director of Bourbon Norway, said he could not speculate on what had caused the accident but confirmed the vessel was carrying out an anchor handling operation with the rig and was connected to it.