The co-pilot of a Heathrow-bound passenger flight had to be dragged from the cockpit and handcuffed after suffering an apparent mental breakdown in mid-air, it emerged yesterday.

Passengers on the Air Canada flight from Toronto to London said the co-pilot was restrained after yelling and "invoking God" while at the controls of the Boeing 767 plane more than 30,000 feet above the Atlantic. The flight was forced to make an emergency diversion to Ireland's Shannon airport early on Monday and the unnamed crew member was taken to a nearby psychiatric unit.

Sean Finucane, one of 146 passengers on board, said the co-pilot was carried into the cabin with his hands and ankles cuffed after he was restrained by cabin crew and a passenger, reported to be an off-duty Canadian soldier. The crew member was then handcuffed to a seat as his captain requested permission to land from Irish air traffic authorities. "He was very, very distraught. He was yelling loudly," Finucane told Canadian broadcaster CBC. "His voice was clear, he didn't sound like he was drunk or anything, but he was swearing and asking for God. He specifically said he wants to talk to God."

It is understood that crew first became concerned less than an hour before they were due to arrive at Heathrow, when the pilot began "speaking loudly to himself and acting in a peculiar fashion", according to one source. The co-pilot was taken by ambulance from the airport to a psychiatric unit. Passengers on flight AC848 were taken off the plane to a hotel while a replacement crew flew to Shannon. The flight arrived at Heathrow eight hours late.

An Air Canada spokesman confirmed that the flight captain had requested a "medical diversion" to Shannon.

"The co-pilot fell ill during the flight and the captain elected to divert to Shannon," he said. "The aircraft landed without incident. At no time was the safety of the passengers or crew in question. The flight was met by medical personnel and the individual is now in care."

A spokesman for Shannon airport said: "We were advised that a crew member was unwell and that was the reason for the diversion."

Members of the co-pilot's family and a representative of the Air Canada Pilots' Association flew to Shannon yesterday.

"Although the illness of flight crew is rare, pilots are fully trained for such an event," said Captain Andy Wilson, president of ACPA. "The safe diversion was the result of the pilot following standard operating procedures in the professional manner that is expected of Air Canada pilots."

Transport Canada, the national body in charge of aviation safety, has opened an investigation and will interview the flight crew. Canadian pilots must undergo a medical check every six months as a condition of their licence.

In 1999 a New York-bound EgyptAir flight crashed into the Atlantic shortly after the co-pilot, who was controlling the plane, was heard to say "I put my faith in God" as the autopilot was switched off. The plane then plunged into a steep dive and crashed into the sea, killing 217 people.