• 37% of voters have lower opinion of prime minister than at start of month • 39% have more positive view of Labour leader, says Observer/Opinium research

Jeremy Corbyn has dramatically cut Theresa May’s previously commanding lead in approval ratings among voters, according to the latest Opinium/Observer poll, in a further sign that the race for Downing Street may be tightening with 10 days to go until the general election.

The narrowing of May’s lead suggests her decision to call a snap election and then focus her campaign almost entirely on her leadership, contrasting it with Corbyn’s, may be backfiring.

More than a third of voters (37%) say their opinion of the prime minister is more negative than at the start of the campaign, against 25% who say it is more positive.

The opposite is true of the Labour leader, with 39% saying they have a more positive view of Corbyn compared with 14% who now have a more negative view.

In the middle of April, the prime minister had a massive 56-point lead over Corbyn in net approval ratings (when the percentage who disapprove is subtracted from those who approve). Now that has been cut to 22 points, a still significant advantage but one that is diminishing at a rate that will alarm Tory strategists.

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Overall, the Tory lead stands at 10 points, a drop of three since last weekend. In mid-April, Opinium put the Tories 19 points ahead of Labour. The Conservatives are on 45% (down one point on a week ago) while Labour is up two at 35%.

The latest signs of a Corbyn bounce come amid evidence of internal Tory disquiet at the conduct of the campaign. The former chancellor George Osborne, who was sacked by May and is now editor of the London Evening Standard, said the prime minister had joined Corbyn in a “retreat from international liberalism and globalisation”.

Osborne denied that he was taking “revenge” on May, pointing out that she had brought about a shift that many of his paper’s readers were nervous about. “That is quite a development in British politics, and I think there are quite a lot of people who are uncertain whether that is the right development and I want to make sure that the Evening Standard is asking on their behalf questions about that.”

He stood by the paper’s criticism of the Conservative manifesto plans on social care, which were swiftly changed last Monday. “They were clearly badly thought through, because the prime minister herself decided to rethink them,” he said.

Some Tory ministers and candidates are complaining about the difficulties in penetrating a “narrow clique” around the prime minister and calling for new faces to be used. One minister criticised the “cult of personality” around May. “MPs and ministers feel they have been pushed out of the script.”

Some Tories have also been annoyed at what they regard as the anti-business tone of the Tory manifesto, which included a vow to cap energy costs, greater curbs on foreign takeovers and a promise to intervene in failing markets.

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There is also mounting frustration about the difficulties in reaching May and the power of her joint chiefs of staff, Nick Timothy and Fiona Hill. One senior Tory said they were effectively two “deputy prime ministers”.

Robert Ford, professor of political science at Manchester University, said the shifts in the polls, while real, would not necessarily be translated into votes on polling day.

“Many voters approached this election not feeling a strong draw to either party or leader. May’s ratings were inflated by an extended honeymoon which was bound to end sooner or later, while Corbyn was at such a low point he could only improve,” he said. “Now we are at a point where May’s bungling [the U-turn over social care] and Corbyn’s detoxification [through offering popular policies] have altered the balance.

“But there is still good reason to doubt whether this will be as significant as it seems now, as many of the changers may be young people and others whom we may not see on polling day, while those most certain to vote are older voters, who tend to vote Conservative.”

Opinium put the Liberal Democrats on 7%, down one point on a week ago, while Ukip and the SNP were unchanged on 5%.