Britain's aircraft pilots' body said on Wednesday it had reached an agreement with one airline on rules governing armed air marshals, but added it was still against taking guns onto planes.
"We remain opposed to the whole concept of bringing sky marshal guns on board an aircraft. This will not make flights more safe," said Jim McAuslan, general secretary of the British Airline Pilots' Association (BALPA) in a statement.
BALPA said it would cooperate with stepped-up security measures announced after six Air France flights between Paris and Los Angeles were cancelled last week and amid fears of an incident similar to the September 11 attacks on the United States in 2001.
"We have always said that if the government persists with its decision that sky marshals be used and if our very serious concerns are met, we shall cooperate as best we can," McAuslan said.
Under the agreement, with an unnamed airline, the captain of a plane will be told who the air marshals are and where they sit. The pilot, who will remain in command during the duration of the flight, will also be in regular contact with the guards.
The pact also covers issues such as the weapons that can be used, insurance cover and the crew's legal liability, BALPA said.
The organization, which represents the vast majority of Britain's 9,200 airline pilots, said it hoped to roll out the pact across the airline industry.
An announcement by the United States on Monday that it was ordering foreign airlines to place armed marshals on selected flights drew a mixed reaction from international carriers and airline bodies.
The U.S. Department of Homeland Security said the measure went into effect on Sunday under emergency amendments to federal aviation regulations.
Britain appeared to anticipate the ruling, saying at the weekend that it would put armed police on flights which could become possible terror targets.
While most airlines publicly fell into line with the decisions, some have expressed their reservations about the policy in the past.
The International Air Transport Association, which represents around 95 percent of airlines operating international flights, has opposed the decision outright.