British Prime Minister David Cameron has said he would resign if he is unable to deliver an in/out referendum on Britain's European Union membership by 2017, a threat that will complicate any coalition talks that may follow next year's election, Reuters reported.

Cameron's comments, made on Monday, April 28 evening during a conference call with party supporters, were aimed at dispelling doubt among voters that his Conservative party is committed to re-negotiating Britain's ties with the EU, and then winning public approval for the reformed relationship with a referendum.

They also come as the anti-EU UK Independence Party is threatening Cameron's Conservatives with serious losses in the upcoming European Parliament elections.

The comments confirmed that Britain's future in Europe will be a sticking point in any future government-forming talks if next May's vote does not produce a clear winner, Reuters says. Britain's current junior coalition partners, the pro-EU Liberal Democrats, are staunchly opposed to Cameron's referendum plan.

"This is not something I would ever barter away or give away," Cameron said on the call, according to British media reports which were confirmed by the party. "I would not continue as prime minister unless I be absolutely guaranteed this referendum will go ahead on an in-out basis."

The Conservatives trail the opposition Labor Party by around 4 percentage points in opinion polls. Labor are also pushing for reform of the EU, but have said they will only hold a referendum if there is a major transfer of power between Britain and the EU - something they don't foresee before 2020.