It was Cameron’s first parliamentary defeat since he was re-elected in May with a slim majority.

British Prime Minister David Cameron suffered an embarrassing defeat in parliament on Monday after Euroskeptic members of his Conservatives joined forces with opposition lawmakers to reject proposed rules for a European Union membership referendum.

It was Cameron’s first parliamentary defeat since he was re-elected in May with a slim majority and highlighted the historic splits over Europe in his Conservative Party that contributed to the downfall of two of his predecessors.

Cameron, who plans to reshape Britain’s relationship with the bloc ahead of a membership vote by the end of 2017, has said he wants to stay in a reformed EU but rules nothing out if his renegotiation is unsuccessful.

While overall legislation paving the way for the referendum easily passed to parliament’s upper house following a vote in the House of Commons in the early hours of Tuesday, the government will now have to accept changes after lawmakers voted 312-285 on Monday against some of the bill’s proposed rules.

Cameron made two concessions last week over the referendum, agreeing to make the wording of the question more neutral and to accept some limits to government activity in the run-up to the vote.

While lawmakers from Cameron’s party back the idea of holding the referendum, many Euroskeptic Conservatives argued the concessions did not go far enough and said a full period of “purdah”, barring the government from publishing anything that could influence the outcome, must be applied.

“If the public detect that the referendum has been rigged to help one side, the public will not feel it’s legitimate, they will not feel that the debates are straight and whatever the result is, many of them will not accept it,” Conservative lawmaker and former minister Owen Paterson told parliament.