Some flights to the US could be grounded after the airline pilots' union called on its members not to fly with armed sky marshals on board.
Airport security has increased
Airline pilots should not take off with marshals on board, the British Airline Pilots' Association (Balpa) has said.
UK ministers say the move is a "responsible and prudent" response to the heightened terror alert in the US.
But the union wants assurances about the captain's powers, the training of marshals and the weapons used.
Balpa has also called for an emergency world summit of pilots to consider the plans by the US to demand marshals be used on some international flights to and from the country.
The UK's marshals were expected to begin working on some transatlantic routes on Monday or Tuesday.
The union wants to meet Transport Secretary Alistair Darling to secure
assurances about the safety of the initiative.
Balpa's announcement came after consulting lawyers to see if pilots could legally refuse to fly their plane with an armed police officer on board.
Balpa chairman Captain Mervyn Granshaw said: "The minister said
yesterday, rather belatedly, that captains will be told when armed guards are on
board their aircraft.
"We want to hear that the captain will be the one in command of the aircraft
at all times, we seek reassurances about the weapons to be used and the training
"We want a written
protocol, an agreement, on when and how armed guards should be deployed."
Balpa general secretary Jim McAuslan claimed the government proposal was clearly in response to the US demand and this needed a worldwide response from pilots.
"We are calling for a world pilot summit through the International Federation
of Air Line Pilot Associations," he said.
"We shall spell out why putting armed guards on
aircraft rather than stopping armed terrorists on the ground is far from a
Capt Granshaw defended pilots' right to take action and said: "Our advice to pilots is that until adequate written
and agreed assurances are received, flight crew should not operate flights where
sky marshals are carried.
Critics fear air marshals may fuel passenger anxiety
"If assurances are not forthcoming we strongly believe it is within the
flight crew's rights to refuse to operate the flight on the basis of the safety
of the aircraft, its passengers and crew as well as the uncertainty about the
legal position of flight crew.
"We have had detailed discussions with our lawyers overnight, and these have
continued today. Their advice is that captains are within their right, as
commander of the craft, not to fly."
In announcing the move on Monday, Mr Darling stressed the best control was to prevent potential hijackers boarding planes in the first place.
But he told BBC Radio 4's Today programme air marshals were "the last line of defence".