The British Air Line Pilots Association (BALPA) is to meet transport secretary Alistair Darling today to discuss how plans for air marshals on transatlantic flights should be implemented.

BALPA has refused to support the introduction of marshals unless pilots are briefed on who they are and where they are sitting, and if the captain can question them before take-off.

Jack Straw, the foreign secretary, said it was essential that a balance was struck.

"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 largest holiday carrier, Thomas Cook Airlines, has declined to accept sky marshals on its flights.