Labour said Parliament must be recalled if Mr Cameron was considering a military response, but Downing Street sources said this may not be necessary as the Prime Minister retained the right to act urgently if required.

Any retaliatory attack would be likely to be launched from the sea as the Syrian air force is judged to be strong enough to shoot down enemy jets.

A Royal Navy nuclear-powered submarine is said to be in the region while a number of warships recently left Britain for exercises in the Mediterranean.

Commanders may also need to make use of the RAF base at Akrotiri, Cyprus for air support.

If military action is approved, the first wave of missiles could start within a week.