Labour has drawn level with the Tories, but Jeremy Corbyn’s personal approval rating has slumped, according to the latest Opinium poll for the Observer.

It suggests that the Conservatives have lost the two-point poll lead recorded by the company last month, with both parties on 40% support. However, the Labour leader’s net approval rating – the difference between those who approve and disapprove of the job he is doing – has fallen from -10 to -19.

The fall follows a difficult period for the party, after it faced demonstrations against antisemitism and Corbyn apologised for appearing to defend an antisemitic mural. Meanwhile, Theresa May’s approval rating remains relatively stable, improving one point to -8.

May’s lead over Corbyn, in terms of who voters think would make the best prime minister, has risen from 8 to 12 points in the last month. She is now on 36% to Corbyn’s 24%.

In the wake of her response to the Salisbury poisoning case, May appears to have increased the number of voters who see her as decisive – up from 32% last month to 41% this month. It gives her a large lead over Corbyn on the question, who has 34% support. May also has a 14-point lead over Corbyn when voters are asked who will “stand up for Britain’s interest abroad”.

Half (50%) approve of the way May has responded to the Salisbury attack, up from 41% last month. Asked which of the two main party leaders would be best placed to respond to Russia and its likely involvement in an attack on British soil, 42% said May while only 16% opted for Corbyn.

However, Corbyn has a significant 18-point lead when voters are asked which leader is “in touch with ordinary people”.

More voters believed Labour was divided than believed the Conservatives were split. The poll also suggests that the Lib Dems remain on single figures of support, recording just 7% backing – up a single point on last month. Some in the party believe that pressure may increase on party leader Vince Cable should it continue to struggle to make an impression in May’s local elections.

The poll was conducted online among 2,008 adults between 10 and 12 April.