The Nato secretary-general, Anders Fogh Rasmussen, said the force, drawn on rotational basis from Nato allies, could be in action at “very, very short notice”.

Rasmussen described it as a mixture of regular troops and special forces that could “travel light but strike hard”. It would be supported by air and naval forces as needed.

He declined to say how many troops would be engaged but Nato officials said it would number around 4,000 and would be expected to deploy to any alliance member country within 48 hours..

“It is so that we are ready should something nasty happen,” a senior Nato official said.