British deputy prime minister Nick Clegg would rule out another coalition between his Liberal Democrats and the Conservative Party if the latter insist on calling a referendum on European Union membership.

Prime Minister David Cameron, the Conservative Party leader, has promised to call such a vote by 2017 if he wins the election in May.

“This is pretty much a red line for Nick,” an unnamed source close to the party leader told the British tabloid newspaper The Sun.

There is no way he will sign up to the plans that will allow the Tories to set the terms of a renegotiation, carry it out and then hold a referendum that could see Britain leave the EU.

The Liberal Democrats favor deeper European integration. Many Conservatives are Euroskeptic. Cameron’s promise to seek concessions from other member states to take Britain out of “ever-closer union” is widely seen as an attempt to draw dissatisfied rightwingers back from Nigel Farage’s United Kingdom Independence Party which advocates leaving the European Union altogether.

The Conservatives are polling neck and neck with Labour while UKIP is hovering around 15 percent support.

Under Britain’s first-past-the-post voting system, a strong showing for the Euroskeptics could allow Labour to win in constituencies where there is a split right-wing majority, denying Cameron reelection.

The Liberal Democrats are projected to lose over half their seats. Many of its voters are more leftist than liberal and haven’t forgiven it for propping up a Tory government that cut public spending and raised university tuition fees.

Clegg, a centrist, convinced his party to join the Conservatives in government after neither they nor Labour won an outright majority in the 2010 election.