Theresa May is under increasing pressure from her own ministers to retaliate against Russia for its “brazen act of war” in using nerve gas on the streets of Britain.

The Defence Secretary Gavin Williamson said the Government was being “pushed around” by Russia and “we have to change the way we deal with it” amid calls for fresh sanctions against Moscow and the expulsion of Russian diplomats.

Boris Johnson, the Foreign Secretary, has already threatened new sanctions if Russia is proved to be behind the attack which has left former spy Sergei Skripal and his daughter Yulia in a coma and Sgt Nick Bailey in intensive care.

Yesterday he repeated his insistence that Britain would respond “robustly” against any country found to be behind the attacks.

Downing Street insisted it could not make any diplomatic moves until police had proved who was behind the attempted murders.

A Whitehall source said: “There is a process to go through. These things have to be done in the right order.”