David Cameron has urged his Eurosceptic ministers for unity and harmony as he set out the four conditions for granting Conservatives a free vote on the EU referendum. The prime minister stressed, among other things, that collective responsibility must continued to be followed by pro-Brexit ministers until renegotiation with Brussels has been concluded.

The Tory leader also warned that his top team could not defy him over any other issues, including EU-related business. Crucially, the civil service will be banned from helping Eurosceptic ministers help make their Brexit case in the run-up to the historic ballot, which will be held before the end of 2017.

"Throughout this period, and in its aftermath, we will continue to have responsibility for governing the country and serving the public who elected us. This can only be done effectively if we remain, despite difference on this one issue, a united, harmonious, mutually respectful team," Cameron said.

The prime minister's letter comes after IBTimes UK exclusively revealed that Leave.EU, one of the two Brexit campaigns, had signed up more than 600 Conservative councillors. Steve Baker MP, co-chairman of the Conservatives for Britain chairman, has previously said that he expected more than 100 Tory MPs to join the rival Vote Leave campaign.

But so far no current Eurosceptic cabinet ministers have been drawn on how they will campaign ahead of the referendum. Work and Pensions Secretary Iain Duncan Smith and Home Secretary Theresa May are two of the frontbenchers expected to back a Brexit.

The latest opinion poll from YouGov, of more than 900 Londoners for LBC Radio, put "remain" eight points ahead of "leave" (54% versus 46%, respectively).