The Conservatives have recorded their biggest lead since the last general election after support for Labour slumped by six points, according to the latest Opinium poll for the Observer.

Theresa May’s party recorded a seven-point lead over Labour in the poll, its biggest since the disastrous election campaign that left her without a majority and relying on the support of Northern Irish DUP MPs.

Labour’s support fell from 40% in the last poll to 34%, while Tory support went up from 37% to 41%. It comes despite continued infighting within the government over Brexit, including a record parliamentary defeat for the prime minister over her proposed deal.

The latest Opinium poll suggests that Labour has lost support from both sides of the Brexit debate. Labour has dropped five points among both remainers and leavers. For the first time since the election, less than half of remainers (49%) would opt for Labour.

Approval for May’s handling of Brexit had increased slightly, while support for Jeremy Corbyn’s handling of the issue has slumped to an all-time low. May’s approval ratings on Brexit edged up slightly to -30%, with 25% approving and 55% disapproving. Her rating had been -33% a fortnight ago.

Meanwhile, Corbyn’s net rating on the issue is now -44%, with 16% approving and 61% disapproving. His rating was -40% in the last poll a fortnight ago. Only 42% of current Labour voters approve of the way Corbyn has responded to the government on Brexit, while a quarter (26%) disapprove.

There were small signs to encourage No 10 about support for May’s Brexit deal. While the deal is still seen as bad, outright opposition has dropped off in the past weeks. Currently, 15% think May’s deal is good, while 45% think it is bad. In the last poll, only 12% thought it was a good deal and 50% thought it was bad.

Most people would back staying in the EU over May’s deal. If a referendum were held with a choice between the two, 45% would vote to Remain, while 38% would vote for the deal. More than half (55%) of Conservative voters now think their MPs should back the deal, up from 48% last week.

The poll shows that voters remain deeply divided about what to do if MPs fail to agree a deal. In those circumstances, 43% think Brexit should be delayed and that a public vote should be held on what to do next. However, 42% think the UK should simply leave the European Union with no deal in March.

Polls have been largely stable in recent months, with the two main parties remaining almost neck and neck. Last month’s Opinium poll showed Labour enjoying a three-point lead. More polling will be required to see if there has been a reliable and sustained shift.

The latest poll was conducted among 2,008 people online between 30 January and 1 February.