However, scores of Conservative MPs are thought to have refused to sign the letter, which will intensify concerns in Downing Street that Mr Cameron’s most vocal opponents could still attempt to launch a vote of no confidence in the Prime Minister in the weeks after the vote.

The view in Labour HQ is that, if Britain does vote to leave, Jeremy Corbyn should call on David Cameron to resign, but senior figures believe that that may prove unnecessary because Cameron may announce his departure of his own accord.

In a speech several hours after most results were counted, Nigel Farage later said Mr Cameron should resign "immediately" if Brexit won what was still expected to be a knife-edge vote.

Eurosceptic Tories on Thursday began speaking about their “duty” to ensure “political stability” before the polls had even closed in what is only the third nationwide referendum in UK history.