Boris Johnson’s Partygate scandal will be a major problem for the Conservatives at the May local elections, according to one of the country’s top political scientists.

       Sir John Curtice said polling had got worse for Tories since the prime minister was fined for his law-breaking birthday bash, adding the saga had “significantly damaged” his party.

       “The story is not going to go away, the story has now got legs again,” the polling guru told The Independent. “The opposition is just going to keep on talking about it. All of this plays to Keir Starmer’s advantage.”

       Professor Curtice said Labour had stretched its lead over the Tories by three points since Mr Johnson was handed a fixed penalty notice by the Metropolitan Police – leading to an average Labour poll lead of eight points.

       Asked if Partygate will be a major problem come the elections on 5 May, Prof Curtice said: “Yes. The government can keep saying we’ve got these other issues. But they are getting bad press over cost of living. And Ukraine is a mixed blessing.”

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       The Strathclyde University professor added: “There has never been evidence of a rallying factor [for the Tories] from Ukraine, and the government is getting terrible press over the visa refugee policy.”

       However, Prof Curtice said much of the public outrage over Partygate has been “largely baked in” since December and January, when the most significant shift in the polls occurred.

       “The latest YouGov poll shows 78 per cent think [Mr Johnson] lied. They thought that in January,” Sir John said. “Stories over the next couple weeks will just reinforce those views.”

       The expert added: “My view is that Partygate has significantly damaged the Conservatives’ standing, though it has not led to the collapse in the Conservative vote.”

       One recent study predicted that the Tories could lose up to 810 council seats at next month’s elections. Around 5,000 seats are up for grabs at local elections taking place across England, Wales and Scotland, while Northern Ireland is voting on the Stormont assembly.

       The Electoral Calculus and Find Out Now study, based on extensive polling in England and Wales, also estimated that Labour could gain up to 835 seats.

       Although Prof Curtice said seat forecasting for local elections was a “mug’s game”, the polling guru said he did expect the Tories to lose a lot of council seats at elections in Scotland, while Labour may seize control of more councils in London – including Barnet and Wandsworth.

       Even if Mr Johnson survives a push on his leadership by restless Tory backbenchers after the local elections next month, he faces another major test with the Wakefield by-election.

       Tory MP Stephen Hammond has suggested that contest, now expected to take place in June, could be the final straw for Mr Johnson.

       On the key test in red-wall territory, the former minister said: “If we don’t [win], there might be some thought about what we need to do to reassure those voters that came to us for the first time in 2019.”

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       Senior Tory MP Mark Harper, who has sent a letter of no confidence to the 1922 Committee, said he expected the 54 letters needed to trigger a vote to be submitted before parliament’s summer recess in July.

       Prof Curtice said: “I think we’ve reached a point where the opposition probably thinks that Boris Johnson staying on is to their advantage. The Labour message is the ‘Tories want to hang onto a lawbreaker’ and they will keep on repeating it.”