The British airline pilots` union is demanding talks with ministers on plans to put undercover armed marshals on flights this week.
News that marshals are being deployed pre-empted a US demand that foreign planes carry “a law enforcement officer” at certain times.
UK ministers say their move is a “responsible and prudent” response to the heightened terror alert in the US.
But the pilots` union says staff might refuse if guns are allowed on planes.
The undercover marshals are expected to begin work on Monday or Tuesday.
The Department for Transport would not say whether or not the move was linked to the US demand, which will apply to specific flights.