LONDON — U.K. Prime Minister David Cameron said he won’t lead a government that can’t deliver a referendum on British membership of the European Union by the end of 2017.

Cameron, seeking to shore up his support among euro-skeptic voters before elections to the European Parliament later this month, said he would hold the referendum even if he hadn’t completed the renegotiation of Britain’s membership of the bloc.

“Whatever the outcome of the next election, people should be in no doubt that I will not become prime minister unless I can guarantee I can hold that referendum,” Cameron said in an interview with BBC Television’s “Andrew Marr Show.” “We will have the referendum whether or not I have successfully negotiated — I believe I will be successful.”

Cameron’s Conservative Party is being squeezed in the polls by the U.K. Independence Party, which campaigns for British withdrawal from the EU, and he has sought to position his party as the only one that has the ability to drive reform. A Survation poll for the Mail on Sunday of voting intentions in the May 22 election published today put UKIP on 32 percent, with Cameron’s Conservatives on 21 percent behind the opposition Labor Party on 28 percent.

UKIP’s support has been driven by anger over EU immigration to the U.K. and the prime minister said the 700,000 net migrants who had come to Britain from the EU since 2004 have “changed our country.” UKIP leader Nigel Farage has said parts of Britain are “like a foreign land.”

Cameron reiterated a proposal to tie freedom of movement within the EU to per capita income in countries which join the bloc. Such a plan would stop a repeat of the numbers who moved from Poland for higher wages in 2004, he said.

“You could have transitional controls that say, for instance, that you don’t have the freedom to move and get a job in another country until, say, your income per capita is at a certain level,” he said. “That would be a way of avoiding some of the difficulties in the past.”