Sky marshals on airliners are a necessary step, the transport secretary has said.

Alistair Darling told MPs that the government had not taken the decision lightly and was doing everything it could to ensure the security of passengers.

His comments came in an urgent statement to the Commons on Tuesday, following the announcement that armed sky marshals would be onboard some flights to the United States.

"In response to the present high state of alert in the United States, additional security measures on the ground and in the air have been put in place for UK airlines operations," Darling said.

"This is judged a responsible and prudent step at the present time but the continuing need will be kept under review."

Shadow transport secretary Theresa May called for greater clarity from the government.

"In the last few days it has emerged that there is some suggestion that the government's decision was only taken because of pressure from the United States," she said.

"People who will be travelling on UK airlines - both staff and passengers - deserve to know as much as possible."

John Thurso, the Liberal Democrat transport spokesman, supported the concerns of airline pilots union over the issue.

"The efforts to prevent terrorist acts on aeroplanes should and indeed must remain primarily focused on ground security," he said.

"BALPA are quite right to be extremely concerned at the potential risk of introducing weapons onto planes."

Darling's statement was later followed by a meeting with airline pilots who demanded reassurances from the government.

The pilots union BALPA expressed alarm at the demand from the US that armed sky marshals should be put on flights to the US.

Officials were reassured that pilots would remain in charge of their aircraft but called for a "gold standard" across the industry and a meeting between industry chiefs, ministers and pilots.

"We will maintain our argument and then, once circumstances change, we hope to persuade the government to step down their policy," said general secretary Jim McAuslan

They also attacked the way news of a massive change in air travel was released.

Ahead of the meeting, the union warned any measures would have to be appropriate to the risk and not token gestures designed to create a false sense of security.

"We shall spell out why putting armed guards on aircraft rather than stopping armed terrorists on the ground is far from a universal solution," said McAuslan.

"It was, after all, not the absence of sky marshals that caused the tragedy of the twin towers but the sloppy security checks on the ground."

Darling will have to settle a series of key demands or face the prospect of a boycott of flights.

"We seek reassurances about the weapons to be used and the training given, we want to know that the government will insure the lives of flight crew and cabin crew when armed guards are put on board, and we want a written protocol on when and how armed guards should be deployed."

Officials at the Department of Transport played down the significance of the meeting.

"The meeting will continue ongoing discussions but obviously has more significance in light of recent concerns," said a spokesman.

Jack Straw, the foreign secretary, said it was essential to strike a balance.

"Getting this balance right is difficult," he said.

"We do our very best but, of course, where there is a fine judgment to be made, we come down on the side of passenger safety and no one would forgive us if we failed to do that."

Britain's biggest holiday firm has expressed alarm at the plan and will be watching the outcome of the meeting closely.

"For the time being, if a sky marshal presents himself, we will cancel the flight," said a spokesman for Thomas Cook Airlines.

"Like BALPA, we want to see captains remain in full control of the aircraft at all times."