An investigation was under way last night after a Scottish aircraft was forced to make an emergency landing when a man brandished a knife on board.

Reports suggested the man threatened to harm other passengers on the plane, which was leased to a Bangladeshi airline but crewed mainly by staff from the Edinburgh-based low-cost company Flyglobespan.

A spokesman for the airline which also operates from Glasgow and Aberdeen raised concerns over airport security after the man was seen with what police described as a four inch knife during the flight.

The Flyglobespan spokes-man insisted there was no violence during the flight but because the man continued to act "disorientated and confused" after being approached the decision was taken to divert and land.

Images taken on the plane showed a man being wrestled by uniformed staff.

Flyglobespan said it was not flight staff but possibly ground security staff crew who tackled him.

There were no injuries aboard the GMG Airlines flight, which was carrying about 70 passengers from Kuala Lumpur in Malaysia to the Bangladeshi capital of Dhaka, and which made an emergency landing in Bangkok, said Watcharapol Prasanrachakij of the Thai Police.

The Thai authorities boarded the plane at Bangkok's Don Muang airport and removed the passenger, who was later identified as a Bangladeshi citizen.

The plane continued to its destination after a delay of several hours in Bangkok. Thai police said the man was due to remain in custody until the next available GMG flight to Bangladesh, when he would be turned over to airline security personnel aboard the plane.

Police later also described the man as mentally unstable.

A spokesman for Flyglobespan said last night: "The staff said in the report that they saw this guy peeling an apple with a knife, so spoke to him but he would not engage with them.

"He was disorientated and confused.

"At that time they decided that because he wouldn't engage with them he was a potential threat and they reported that to the captain who decided to take the action he did.

"There wasn't any great threat to the crew. The guy wasn't responding to what the crew were saying and so they diverted into Bangkok airport and he was taken off the plane.

"It was a Flyglobespan aircraft which has been on lease to the company out there to GMG Airlines since December.

"The flight deck - the captain and the first officer - are Flyglobespan personnel and the senior cabin crew member was Flyglobespan personnel with the remainder of the cabin crew being local. The majority of the crew were local."

The plane was a Boeing 737 800, which Flyglobespan said can hold around 170 passengers. Initial reports claimed a fight had broken out on board the flight.

The spokesman went on: "Clearly the departing airport will need to look at its security.

"No matter how small the knife was he shouldn't have got it on board."

He added that the crew members were "resting" last night and unavailable for comment. "The ops report had nothing about the guy being violent or having to be restrained. But that is only up until the point the plane landed.

"Once the plane lands it becomes the responsibility of the airline (rather than aircraft cabin crew) and it may well be this picture taken is of one of the airline or (airport) security people."