Labour has opened up its biggest poll lead over the Conservatives since shortly after last June’s general election, as the government’s chaotic handling of Brexit appears to have triggered a dramatic fall in support for Theresa May and the Tories – and a sudden revival of Ukip.

The latest Opinium poll for the Observer puts Labour on 40%, the same score as last month, but four points ahead of the Tories who have dropped by six points since early June to 36%. The fall in Tory support would appear to be the result of Conservative supporters who backed leaving the EU turning to Ukip, whose support has shot up by five points from 3% last month to 8%.

May’s leadership ratings have also nosedived, while those of Jeremy Corbyn have remained stable. May’s net approval rating – when the number of people who disapprove of the way she is leading the country is subtracted from the number who approve – has tumbled from -8% in June to -24% over a turbulent political period of just five weeks.

She is now well behind Corbyn who is on -12%, little changed from -13% last month.

Overall the poll findings suggest that the turmoil in government caused by Brexit, which led last week to the resignations of foreign secretary Boris Johnson and Brexit secretary David Davis, is having a more significant effect on how the public view the different political parties and their willingness to support them, than has been the case so far. About 51% of people now think Brexit is one of the most important issues facing the country – the highest percentage recorded so far by Opinium – and up markedly from 42% last month.

May now has the lowest approval rating specifically on her handling of Brexit since Opinium starting polling on the issue. Just 25% currently approve of the way she is handling Brexit, down from 30% last month, while 56% disapprove, up from 45% last month.

Public opinion is completely split on Brexit, mirroring divisions within the main political parties. Asked whether they approved of the deal hammered out at Chequers between cabinet ministers last weekend, 32% of respondents said they backed it while 32% opposed it.

The public’s view of Corbyn on Brexit has not shifted much since June, with 22% approving of his handling of the process of leaving the EU, up from 21% last month. In terms of net approval on Brexit, May now also fares worse than Corbyn with a net approval of -31% compared with a net approval of -23% for the Labour leader.

Half of voters think the electorate should be given another referendum on any final deal that is bought back from Brussels by May, against 33% who do not.

People are split, however, on whether the choice on the ballot paper should be between accepting the deal on offer and leaving without any deal at all or between accepting the deal on offer and staying in the EU, both supported by 25% of those asked.

While Labour’s poll lead will encourage some in the party to push for an early general election, others in the party will be worried that its support has not increased at a time when the Tories are in open revolt and lack any unity over Brexit. Neither have the pro-EU Liberal Democrats benefited from Tory woes. They are up only one point to 8% since June.

The figures suggest large numbers of leave voters have become disillusioned with the Tories’ apparent drift to a more soft-Brexit position, and are looking to Ukip as a way to deliver a clean break with Brussels. Among potential Tory replacements for May, were she to step down or be forced out, people seem to have no clear preference but Johnson is favoured by more people than any other.

Asked who they think would be the best prime minister among the likely Tory runners and riders, Johnson comes out top with 12%, followed by Jacob Rees-Mogg (10%), David Davis (7%), Philip Hammond (5%), Jeremy Hunt (4%), Michael Gove (3%), and Andrea Leadsom (2%). About 25% said “none of these” and 22% said they did not know.