A crew member airlifted from the Canadian submarine drifting without power off the north coast of Ireland to Sligo General hospital, is in a critical condition tonight.
The sailor was airlifted, along with two other crew members, by a Royal Navy search and rescue helicopter, but his condition deteriorated soon afterwards.
All three crew members suffered smoke inhalation when an electrical fire broke out on board the vessel yesterday.
A spokeswoman for the hospital tonight said: "The three men landed on the helicopter deck at the hospital almost an hour ago and were rushed in - one of them is in a critical conditions and the other two walked in. Doctors are seeing them just now."
The Ministry of Defence said the Sea King had planned to take the crewmen to Derry for medical treatment, but had been forced to make an emergency landing at Sligo.
"The condition of one of the men deteriorated and gave us cause for concern," said a spokesman. "A medical decision based on his condition was taken to put down in Sligo. The helicopter landed as quickly as it could."
RFA Wave Knight, HMS Montrose and HMS Marlbrough are helping in attempts to restore power to the vessel, but are expecting to be hampered by diminishing light and "very rough seas", according to a British MoD spokesperson.
Two Irish ships are, meanwhile, en route to assist the Canadian submarine, which lost power after the fire knocked out its diesel engines and injured nine crew.
HMCS Chicoutimi, one of four trouble-plagued second-hand submarines that Canada bought from Britain, was on its way to Halifax with 57 people on board when the fire broke out in an electrical panel.
A Defence Forces spokesman confirmed the LÉ Aoife and the LÉ Niamh, both of which were patrolling off the south coast, are on the way to the scene. However, he said that because of bad weather in the area, it could be "some time" before they reach the submarine.
A Royal Air Force Nimrod aircraft is understood to be in the area and monitoring the situation. Helicopters are also on standby in Derry.
The LÉ Róisín was damaged after she went to the assistance of the stricken submarine last night. The ship has anchored in Donegal Bay to carry out repairs to a damaged area of its hull.
The Irish coastguard is not directly involved in the operation as the submarine is not in Irish coastal waters.
A senior Canadian official said a towing vessel would take between 12 to 14 hours to reach the submarine. "The boat is without power and has only limited means of communicating at this time," he said.
"The seas are rough and with the submarine on the surface they [the crew] are going to have some discomfort from rolling and tossing," the spokesman added; he denied there was any danger of losing the craft.
Although the fire was quickly put out, the submarine had to surface to get rid of the smoke. Nine crew members suffered the effects of smoke inhalation.
The Naval Service said the LÉ Róisín suffered "significant damage" to her bow area in heavy weather conditions when it was proceeding to assist the submarine last night.
"Some of the supports of the exposed bow area of the ship were ripped out by an unusually large wave in a confused sea, pulling a hole in the deck area," a statement said.
"Consequently the ship was forced to return to Donegal Bay to effect temporary repairs. The damage is currently being assessed by ships marine engineering staff, and on completion of temporary repairs the ship will return to Haulbowline Naval Base in Cork.
"The Navy has dispatched two other ships that were on fishery patrol off the south coast to stand by the submarine."
The Chicoutimi is non-nuclear powered and carries no nuclear warheads.
The fire is the latest in a long line of problems to hit the submarines, which have been plagued by serious mechanical mishaps such as cracks in the diesel exhaust valves and a number of leaks. They are mostly confined to port.
Ottawa insists it got a good deal when it agreed to buy the second-hand submarines from Britain for 750 million Canadian dollars in 1998. But opposition legislators say the purchase reflects incompetence by the Liberal government.
The Green Party's marine spokesman, Mr Eamon Ryan, paid testimony to the courage and skill of the crews of the three Irish boats which have been involved in rescue attempts.
"However, the rescue operation does raise serious questions about our Government's refusal to invest in an ocean-going emergency tug vessel, recommended in an independent report five years ago - which is why the Canadian submariners will have to wait 14 hours for a British towing vessel to reach the scene."
"The Green Party has repeatedly raised concerns with both the Taoiseach and the Minister for the Marine about their decision not to invest in an ocean-going emergency towing vessel. Hopefully, the events of the last two days will lead to the Government reversing its decision. At the very least, we need a full investigation into how prepared, or otherwise, we were for the unfolding events."