The Sea Kings were based on the Ark Royal

Prince Andrew, who flew Sea Kings in the Falklands War, sent his condolences to their families, adding that he was "shocked and deeply saddened".

The Ministry of Defence said enemy action was not responsible for the loss of the six British servicemen and a US officer on board the surveillance aircraft.

All of those killed were thought to have been based at the Royal Naval Air Station at Culdrose in Cornwall, where flowers were laid and the flag lowered to half mast.

The incident happened a day after eight UK troops and four US marines died when a US CH-46 Sea Knight helicopter crashed in northern Kuwait.

It was so badly damaged that it was hardly recognisable as the parts of a helicopter Lieutenant Mark Campbell HMS Liverpool

Following the collision, a statement from Buckingham Palace said: "As a helicopter pilot with combat experience, the Duke wishes to send his prayers and thoughts to the families of the bereaved."

A spokeswoman from RNAS Culdrose said: "The fleet air arm is a close community. As you can imagine we are all saddened by the situation."

Base commander Captain Mike Knowles said: "There is a large family at Culdrose and that family actually extends out to all the ships that are serving and so everybody is going to be affected by this... it really is very, very sad news."

Downing Street expressed "deep sadness" at the loss of more military personnel.

'Tragic accident'

The Sea King Airborne Early Warning aircraft, which had been involved in surveillance work for UK troops, crashed in international waters five miles from the Ark Royal at about 0430 local time (0130 GMT).

One had been leaving the Royal Navy flagship on a mission as the other returned from the same operation.

Sea King AEW Early warning role since 1982 Detect low flying attack aircraft 300 in UK service Full Sea King fact file HMS Ark Royal fact file Dangers of wartime flying

Witnesses on the nearby destroyer HMS Liverpool said they heard a loud bang and saw "a huge ball of orange fire" exploding above the horizon.

The Ark Royal's Captain Alan Massey said an investigation was under way into what he described as a "tragic accident".

He paid tribute to the "extremely professional" crew members killed, adding: "As you can imagine the loss is being felt very intensely".

A rescue operation was launched within minutes of the crash.

The minesweepers HMS Brocklesby and USS Dextrous searched for survivors and were also told to salvage the aircraft and its black boxes and weapons. Helicopters and dinghies also joined the search.

'Almighty explosion'

HMS Liverpool's Lynx helicopter was the first to the area of the crash.

Pilot Lieutenant Mark Campbell said: "All we could see was debris and it was so badly damaged that it was hardly recognisable as the parts of a helicopter. It was a shocking sight."

Chief petty officer Trevor Horobin, an aviation technician on HMS Liverpool, said: "There was an almighty explosion and I saw a huge orange fireball just above the horizon.

"Then the fireball got smaller and crashed into the sea. It was obvious whatever went up was fully loaded with fuel to cause such an explosion.

Kuwait helicopter crash kills 12 Click for full story

"I thought at first it might have been an aircraft hit by a Tomahawk cruise missile."

Captain Massey said the thoughts of everyone on board the Ark Royal were with the families of those killed, and that the ship's work would continue.

"I know that what they would have said to us now that there is a mission to do... we have Marines on the ground who need our support," said Captain Massey.

He denied suggestions that procedures may have been ignored and said the helicopters had been operating to "peacetime safety" levels, with maintenance, training and sleeping patterns followed properly.