There are 100 Tory politicians that threaten to tear this government apart
What seemed like a good way to stop on-the-fence voters turning to UKIP in the elections, now risks tearing his own party in two.
The Independent reports that 100 Tory MPs have promised to vote against EU membership unless the Prime Minister can radically renegotiate the UK's position in Europe.
We already knew plenty of MPs were threatening to disobey Cameron over his desire to stay in Europe. The Prime Minister was forced into a u-turn yesterday over his declaration at the weekend that ministers couldn't campaign for the UK to leave Europe after outrage from his own party.
But this is the first time we have a concrete number on how many Conservatives are actually lining up against the Prime Minister. The Conservatives for Britain group, formed on Sunday, is calling for the UK to have a veto on EU laws and wants an end to freedom of movement.
If Cameron fails to secure these tough new measures - a serious risk - it could spell disaster for Cameron. The Conservatives have a majority of just 12 in the House of Commons and if any of the MPs that have promised to fight against Europe decide to defect or stage a backbench rebellion, the current government could collapse. This would force a coalition or hung parliament.
Citi warned this week that this scenario is a serious danger, saying both a "yes" and "no" vote could bring down the government. Conservative MP David Davis is also warning that Cameron risks reigniting a "bitter argument" within the Tory party over Europe.
Things won't get any easier for the Prime Minister before the referendum, which could come as soon as next year.