Support for the United Kingdom to leave the European Union stood at 43%, ahead of support to stay within the 28-member bloc, which stood at 39%, according to an online survey conducted by pollster Opinium for the Observer newspaper.

The poll of 1,966 adults, who were interviewed ahead of a referendum to stay in the European Union on June 23, found that 18% of respondents were undecided, while 1% refused to say.

But notwithstanding the survey results, the fate of the UK's membership within the European Union could depend on whether supporters in favour of retaining UK's EU membership status could mobilise young voters. Indeed, survey results showed that those in the 18-34 group, 53% said they backed staying in, against 29% who wanted to leave.

However, the poll results showed that only just over half - or 52% - of respondents in this age group said they were certain to actually go and vote.

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. And according to the Observer, the decision over whether the UK remains inside the EU could now depend on whether young voters shake off their apathy and vote in sufficient numbers.

Contrary to young voters, respondents in the 55-and-pver category were more certain to vote, as 81% said they were certain to vote. Support for leaving was far stronger as 54% agreed with Britain exiting the EU, while only 30% said they wanted the UK to remain the EU, offering a huge advantage to the Leave side.

Adam Drummond of Opinium said the results showed the coalition of support for Remain looked far less solid than that for Leave: “This shows how important turnout levels are going to be, particularly given the disparity between how likely the young and the old are to vote. Young people are much more pro-EU but much less likely to bother voting, meaning that a key element of Remain’s coalition is looking flaky.”

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 UK Prime Minister David 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.