Europe’s refugee crisis has convinced more British people to leave the European Union than to remain a member, according to a new survey.

The YouGov poll, published Monday, showed that 40 percent of respondents supported a withdrawal from the 28-member bloc, with 38 percent opting to stay in.

It was the first time since November 2014 that the “leave” campaign was ahead.

Also, 16 percent did not know how they would vote in the upcoming referendum, while 6 percent said they would not vote at all.

YouGov had shown a lead of 10 points for the “stay” campaign as recently as June, suggesting this summer’s refugee crisis has caused a shift in British public opinion.

Britain is due to hold a referendum on its membership of European Union before the end of 2017.

The precise date will be announced once U.K. Prime Minister David Cameron concludes a renegotiation of Britain’s EU membership terms with other European leaders.

But Cameron has not explicitly spelled out what his negotiation will involve, and critics have said he cannot succeed in obtaining concessions from European leaders on contentious issues such as the free movement of workers.

Nigel Farage, leader of the U.K. Independence Party, which supports British withdrawal from the EU, criticized the prime minister for not negotiating “anything substantial at all”.

He told his party conference on Friday: “He isn`t asking for us to get back control over open borders and the free movement of people to nearly half a million. He isn`t asking for us to get back the supremacy of British law in our own parliament and indeed that our own Supreme Court should be supreme.

“He isn`t even asking that our membership fee of £55 million [$83.6m] a day should be reduced.”

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