Next month's local elections could damage the two-party reign of Labour and the Tories beyond recovery, Sir John Curtice has said.

The polling expert said that if Reform UK and the Greens gained significant numbers of councils and councillors, they would establish activist bases.

The predicted loss of wards by Labour and the Conservatives would cause 'serious damage' to their ability to rebuild, he added.

He also warned that a pro-independence majority in Scotland and electoral upheaval in Westminster in 2029 could spell the end of the union.

In an Ipsos briefing ahead of May 7's elections, the Professor of Politics at the University of Strathclyde said the polls could change the shape of British politics.

He said the results would show whether Reform UK and the Greens were a 'long-term challenge' for the two main parties or a 'short-term bubble'.

Sir John told the event that the question that had hung over British politics since 2024 was whether we were witnessing the end of the two-party system.

'If indeed, the challenge of Greens and Reform is going to be a long-term challenge rather than a short-term bubble, one of the crucial requirements is that these parties develop activist bases and local parties up and down the country, all the way from wick to St Ives,' he said.

He added: 'Assuming that the Greens and Reform do get significant numbers of councils, councillors are the best local activists that any party has.

'So what this will do is it will help to solidify the progress the Greens and Reform are making in creating proper party organisations with activist bases, and meanwhile, the activist base of the Labour and the Conservative parties in various parts of the country are going to suffer serious damage, and that makes the probability of the two-party system recovering lower than it otherwise would be.'

Sir John said that there was already evidence that Reform and the Greens were professionalising, with Nigel Farage's party standing in 99.9per cent of wards and Zack Polanski's in 90per cent.

He said that Reform was standing in more wards than anybody else, while the Greens were fighting more wards that the Liberal Democrats.

The fact these newly popular parties were able to find candidates to stand for them across the country suggested they were already building an activist base, he added.

Reform's support is made up almost entirely of Brexit voters, but he said Mr Farage had been unable to capitalise on this with Ukip or the Brexit Party in the past.

But he said: 'Reform are actually embedding themselves in the politics of the land up and down and the Green are also now in that position.'

He said that while both parties would inevitably be buffeted at some point, they could take lessons from the Lib Dems following their electoral wipeout in 2015.

The way the party survived was that there was 'still one man, one woman and a dog willing to fight local elections and to keep the party alive up and down the country', he said.

After losing 49 of their 57 seats in 2015, they managed to win 72 at the 2024 general election due to this infrastructure, he added.

Sir John also said that if the SNP do well in Scotland and Labour faces the choice of doing a deal with the SNP or being thrown out of office at the General Election in 2029, that is the point where 'deals get made'.

This could lead to the break-up of the union – though support for independence would have to rise as it is still around 50per cent.

Sir John concluded that his overall prediction for a 'difficult' set of elections for the Government was that both Labour and the Tories would lose a 'barrel load' of seats and that many councils would see significant changes of control.