Labour has opened up a three-point lead over the Tories as Conservative Leave supporters appear to be deserting Theresa May’s party in droves, according to the latest Opinium poll for the Observer.

Compared with a month ago, the Tories have dropped five points to 36% while Labour has gained three to stand on 39%. The Liberal Democrats have fallen by one point to 7%, while Ukip has gone up two to 8%.

Opinium found that the Tory decline was primarily a result of Leave supporters deserting the party. Last month (on 11 October) 59% of Leave supporters said they would vote Conservative.

But in Sunday’s poll, conducted entirely after details of May’s Brexit deal were made public last week, the proportion of Leavers backing the Tories has dropped by 10 points to 49%. Labour’s support among Leavers has risen by four points to 26%, while Ukip has surged by six points among Leavers to 16%.

The poll will encourage Jeremy Corbyn’s party as it tries to push for a general election as the means to end the Brexit chaos.

It will be less welcome news, however, for those pushing inside the party to back a second referendum on Brexit, as it suggests Leave supporters are prepared to head towards Ukip if they fear Brexit is being compromised.

The poll found that just 22% of all voters think May’s deal is acceptable. Less than half – 46% – of Conservatives think their party should back the deal being championed by May.

Labour supporters are also divided over whether their party should back the deal, with 42% saying Labour should reject it and 22% saying the party should back it.

When asked if there should be a public vote (a second referendum) if parliament votes down May’s proposed deal, 49% think there should be, while 38% do not.

May is still viewed by more people as the best prime minister though her lead over Corbyn has dropped by two points since last month: 30% back May compared with 23% for Corbyn.

Twice as many voters (56%) disapprove of the way May has handled Brexit, compared with those who approve (26%).

The plus side for May, however, is that Corbyn’s ratings on Brexit are arguably worse: only 19% approve of the way he is handling Brexit against 50% who disapprove.