The BBC's home affairs correspondent Margaret Gilmore explains some of the issues around the introduction of armed air marshals to Britain's transatlantic flights.
The air marshals will only be on flights to the US
Q: Who are the air marshals?
They have been in training for the past year and are all people with a police or military background.
They were trained by the Metropolitan Police in London and are likely to be carrying low-velocity ammunition which should not penetrate the outer skin of the aircraft.
Q: Why have they been introduced?
There has been substantial pressure from America since two planes were hijacked and flown into the World Trade Centre on 11 September, 2001.
Recently they have become increasingly concerned about the possibility of hijackings.
The air marshals are designed to instil fear in the potential terrorist, who won't know which flight they will be on, and to give a kind of reassurance to the travelling public.
Q: Where will they be used?
There are not going to be armed air marshals on run-of-the-mill domestic flights.
They will only be used on flights between the UK and US, but no-one will know who they are because they will be disguised as fellow passengers.
Q: Is there opposition to the plan?
The airlines are very concerned. They have been in negotiations with the government and reluctantly agreed to the measure.
And the British Airline Pilots Association has opposed their introduction on the principle of not allowing any guns on planes. It says pilots might not to fly without knowing when they are carrying the marshals.