In a poll by YouGov for the Times newspaper, 45% of respondents said Britain was wrong to vote out.
43% said Britain was right to leave, while 12% answered ‘don’t know’.
The results come from a poll of 1,590 British adults and suggest the majority has edged against Brexit in what remains a hugely divisive issue.
The question asked: ‘In hindsight was Britain right or wrong to vote to leave the EU?’
The poll also showed that the Tory lead has slipped slightly, with 45% of people saying they intend to vote Conservative in the general election, down 3% on last week.
The government’s loss has been the opposition’s gain, with 29% saying they plan to vote Labour, up 4%, while support for the Liberal Democrats has fallen by 2% to 10%.
Voters are also losing confidence in the Prime Minister, though the vast majority – 48% – believe she would make the best PM, down 6%.
And while Jeremy Corbyn has a 3% increase in support, he is only backed by 18% of people.
The majority of voters are standing by the decisions they made in the referendum, with 85% of those who voted to leave saying they made the right call, while 89% of those who opted to remain saying the result was the wrong one.
David Cameron has defended his decision to call the controversial vote last June, saying the lack of a referendum was “poisoning” British politics.
Speaking at a tourism industry conference in Bangkok, the former PM said: “If Theresa May is successful, she’ll actually have a larger majority and, potentially, more time to deal with Brexit and its consequences.”
He added that the government had “accepted the referendum result and got on with the process of responsibly delivering it”.