Boris Johnson is leading his government’s response to Ukraine “amid fears that a Russian invasion is imminent”.

The foreign office in London has urged UK nationals in Ukraine — and there are several thousand of them — to “leave now while commercial means are still available”, while it has been announced all British military personnel who were in the country to train local forces in the use of anti-tank missiles “will be leaving over the course of the weekend”.

The armed forces minister James Heappey told BBC Radio 4’s Today programme on Saturday that Russia is now at a stage where it can attack “at no notice”, while Tobias Ellwood, chairman of the Defence Select Committee and one of the Tory MPs who wants Boris to resign, predicted “an invasion is imminent”.

However, Malcolm Rifkind, a former defence secretary, told the Rupert Murdoch-owned Times Radio: “I look at Putin not as a wild fanatic; he’s not an Adolf Hitler about to launch all-out war just for the sheer nastiness of it… he knows that he has to carry Russian public opinion with him, and why I’m sceptical as to a full-scale invasion.”

In Kiev, the British ambassador to Ukraine Melinda Simmons tweeted that she would remain behind “and continue to work there with a core team” and that “the embassy remains operational”.

Meanwhile, four American B-52 long-range bombers, which can carry nuclear or precision guided conventional weapons, have arrived at RAF Fairford in Gloucestershire.

This is the background against which a spokesman for the prime minister has confirmed that Boris has indeed received a questionnaire from Scotland Yard and will “respond as required”.