The Duke of Cambridge played a part in a rescue operation in the early hours today co-piloting a helicopter which plucked two men to safety.
One person is dead and another five remain missing tonight as the search for survivors continues.
Rescuers searching for five men missing from a cargo carrier which sunk in the Irish Sea said tonight that a beached lifeboat could hold survivors.
The 81-metre cargo carrier sank 10 miles west of the Lleyn peninsula in north Wales after being hit by an "enormous wave."
William was called into action after the Swanland issued a mayday call at around 2am when its hull cracked.
He has since returned to base at RAF Valley in Anglesey, Wales, the Ministry of Defence said.
The two members of the Russian crew of the Cook Islands-registered vessel were pulled from the water soon after the alarm was raised.
Both were taken to hospital at nearby Bangor as a precaution.
An inaccessible liferaft found near Bardsey Island, off the Lleyn peninsula, may now contain the best hope of finding survivors.
Two attempts earlier today to make certain that nobody was inside the liferaft's protected enclosure were unsuccessful.
A crew member from a Sea King rescue helicopter was winched to within feet of the liferaft but conditions made it impossible to get closer.
An Abersoch inshore lifeboat crew also attempted to access the raft, beached among dangerous rocks, but was also forced to give up.
"The rescue helicopter hovered over the raft and a man was winched down but could not illicit a response," a spokesman for Holyhead coastguard said.
"With a helicopter hovering 50ft above them, and the noise it must have made, it is unlikely that anyone was there.
"But you never know. There is always a chance that somebody might be inside."
He added that wardens on the island had indicated that it was now possible to access the area on foot to check on the liferaft.
Earlier today extra resources were drafted in to the search which covers 300sq miles of the Irish Sea.
A naval vessel from Ireland was taking part in the search and a fixed wing jet aircraft was also added to the rescue operation.
In addition the Holyhead lifeboat crew was searching offshore and a Trearddur Bay inshore lifeboat is searching the sea off the Lleyn peninsula.
"We are trying to make the most of the remaining daylight," the spokesman added.
He said that once night fell a decision on the scale of any continuing operation would be taken.
William is understood to have been involved in the operation today over several hours.
A spokesman for the Ministry of Defence said: "The Duke of Cambridge was a co-pilot on one of the aircraft involved. He is now back at base."
Gale force winds battered the Irish Sea during the early hours of the morning and the Coastguard believe this could have caused the incident.
Holyhead Coastguard watch manager Ray Carson said: "The two men recovered from the water were brought here before going to the hospital. I think they are OK and are just suffering from shock.
"In broken English and through drawing a diagram, the second officer told us the ship was hit by an enormous wave. It rolled the ship and it broke its back. He said this led to a catastrophic failure of the vessel."
Helicopters from RAF Valley and the Dublin Coastguard were involved in the initial operation before they were replaced with helicopters from RAF Chivenor and the Irish Coastguard.
Lifeboat crews from RNLI Phwllheli and Porthdinllaen are among those assisting in the search.
A RNLI spokeswoman said; "Porthdinllaen all-weather lifeboat was launched in gale force winds and rough seas at 2.30am to a cargo vessel in distress with a damaged hull. Phwllheli all-weather lifeboat was also launched.
"A short while later the 80 metre cargo ship sank. A helicopter from RAF Valley rescued two of its crew. The two RNLI lifeboats, along with four search and rescue helicopters and two other commercial boats, are searching for the remaining six crew."
Last August the Swanland came close to going aground on rocks at Lizard Point, Cornwall.
The nine-man crew ship suffered engine failure in high winds as it carried a cargo of stone in the early hours of August 20.
A rescue helicopter was put on standby and a lifeboat was at the scene but Falmouth Coastguard's emergency towing vessel Anglian Princess successfully brought the ship to safety. No-one was injured.
When the emergency vessel arrived a decision was made to caution the ship's skipper that "the Secretary of State's Powers of Intervention may be invoked if the owners failed to take suitable steps to alleviate the situation".
The owners of the vessel, registered in the Cook Islands, then agreed to take the tow less than 1.5 nautical miles from Lizard Point.