The vote is still months away, but the polls are starting to trend toward a Brexit.

In a blow to David Cameron and the pro-EU camp, the online survey by Opinium puts the Leave side on 43%, four points ahead of Remain, on 39%. Some 18% of voters said they were undecided, while 1% refused to say.

While most of the “don’t knows” said, when pushed, that they were leaning towards Remain, offering hope to the pro-EU side, the survey will serve as a wake-up call to leaders of all four main Westminster parties, who are urging people to back their calls for continued membership.

Above all, it will be deeply worrying for Cameron, who will almost certainly have to resign as prime minister in the event of a vote to leave. But it also adds to pressure on Labour leader Jeremy Corbyn. Only 47% of those asked said they identified him as being in favour of remaining in the EU, while 40% said they did not know his view and 12% believed he wanted to leave. Some 78% knew that Cameron wanted to remain in.

Government strategists and pollsters privately admit that the central problem for the Remain side is that its support for staying in the EU is strongest among young people, the group least likely to vote. Opinium found that in the 18-34 age group, 53% said they backed staying in, against 29% who wanted to leave. But only just over half (52%) in this age group said they were certain to actually go out and vote.