More than twice as many people back Theresa May to handle the UK’s response to the crisis in relations with Russia as those who would prefer Jeremy Corbyn to be in charge, according to an Opinium/Observer poll.

The findings suggest the prime minister’s action in expelling 23 Russian diplomats and blaming Moscow directly for the Salisbury nerve agent attack has struck far more of a chord with the British people than the Labour leader’s more cautious response.

In a further sign that May could enhance her standing with continued firm handling of the crisis, the poll conducted late last week, puts the Conservatives two points ahead of Labour, despite deep Tory divisions over Brexit and uncertainty over the country’s economic prospects.

It is the first time since the snap general election last June that the Tories have been ahead of Labour in two consecutive months.

While Corbyn is seen by 48% as a leader who sticks to his principles – compared with 39% who say May does – the prime minister is seen as better equipped to stand up for the national interest.

Asked which of the two leaders would be best at reacting to Russia’s likely involvement in the attack on former double agent Sergei Skripal and his daughter, Yulia, 39% said May and 16% said Corbyn.

The Labour leader has come under criticism in recent days – including from MPs in his own party – for calling on government ministers to avoid “rushing way ahead of the evidence” by pinning definite responsibility for the attack on the Russian state without producing conclusive evidence to back the charge.

The poll shows overwhelming support for May’s decision to expel the 23 diplomats, with 56% of respondents backing the move and only 11% opposing it. Corbyn has supported the expulsion while at the same time insisting that a more effective policy would be to crack down on Russian oligarchs resident in the UK with new anti-money laundering rules.

Overall, the Tories stand on 42%, unchanged since a month ago, while Labour is up one point on 40%. The Liberal Democrats are down one on 6%, while Ukip (down one) and the SNP (unchanged) are both on 4%.

For the first time since the general election May’s net approval rating (-9%) is better than Corbyn’s (-10%).

More than two-fifths of voters (43%) say May has the best interests of the country at heart compared with 39% who say Corbyn has. Thirty-two percent say the prime minister is able to stand up for Britain’s interests against 28% who believe Corbyn is.