The Conservative Party will tomorrow publish draft legislation to guarantee a referendum on the UK's membership of the EU.
It is an unprecedented step. It's the first time since the coalition was formed in 2010 that one of the governing parties has published legislation by themselves.
The Prime Minister is under pressure from his backbenchers to guarantee in law his promise of an IN/OUT referendum on Europe after the next election.
Seventy Conservative MPs have now backed an amendment which "expresses regret" that Europe did not feature in last week's Queen Speech - which set out the government's legislative programme for the next session of Parliament.
Mr Cameron is unable to unveil government legislation because it would be blocked by his Liberal Democrat coalition partners.
Downing Street sources say that the Prime Minister will explore every means possible to get the Conservative draft bill onto the statute book.
That might mean backing a Conservative MP taking the measure forward as a Private Members' Bill. The ballot for Private Members' Bill takes place on Thursday.
But the Tory backbencher, John Baron, who tabled the amendment, which effectively forced the Prime Minister's hand, told ITV News Downing Street has taken the wrong decision and should instead support his amendment.
A senior Liberal Democrat source said:
The Conservatives are free to bang on about Europe as much as they like within their own party.
Rather than spend time debating an internal Tory trauma on an issue where the government has a clear position, the Liberal Democrats will focus on jobs and growth.
The row over how to guarantee a referendum, which is not supported by the Liberal Democrat and Labour parties, followed the Prime Minister to Washington - where he held talks today with President Obama.