Nigel Farage said he believes a "truly historic shift in British politics" has occurred after Reform UK won hundreds of seats and took control of more councils in England.
The Reform leader said his party was showing it could win in Conservative and Labour heartlands, adding he expected the support from voters would not be a "one-off".
The party's gains include winning control of its first London borough in Havering, surging to victory over the Tories in Essex and Suffolk, and picking up seats at the expense of Labour in the Midlands and the north of England.
Reform came second in the Welsh Parliament elections behind Plaid Cymru while party figures said they would be "competitive" in Scotland.
Reform was in its infancy the last time these councils in England were up for election, and when the national contests in Scotland and Wales took place.
The party gained control of 10 councils in England last year and has now followed this up with further successes at the ballot box.
Increased support for Reform saw them take control of councils from Labour, including Barnsley, Wakefield, Sunderland and Gateshead, and shift Hartlepool, Tameside, Redditch and Tamworth to no overall control.
A third of the seats were up for election in Wigan, with Reform winning 24 out of 25 as Labour suffered heavy losses to see its majority on the council reduced.
Control of Newcastle-under-Lyme passed to Reform from the Tories.
Speaking after Reform secured control of Havering, Farage said: "What's happened is a truly historic shift in British politics.
"We've been so used to thinking about politics in terms of left and right, yet what Reform are able to do is to win in areas that have always been Conservative, but equally, we're proving in a big way that we could win in areas that Labour has dominated since the end of World War I."
Farage highlighted his party's successes in former Labour strongholds dubbed the "red wall", which the Conservatives initially made gains in the 2019 general election under the leadership of Boris Johnson before suffering losses in 2024.
He said he was convinced a "fundamental change" was happening in which voters "aren't just coming to us for a one-off, they're now becoming Reformers in every way".
Farage added Reform was "competitive right down from the southwest of England up to the northeast of Scotland".
Polling expert Sir John Curtice said Reform has done best in places that "voted heavily for Brexit" in 2016.
He later outlined a projected national share of the vote for Britain, which suggested Reform was on 26% ahead of the Greens on 18%, Labour and the Tories both on 17% and the Liberal Democrats on 16%.
This was on the basis of results in more than 1,000 wards where the BBC collected detailed voting data and if people who did not have an election voted similarly to those who did.
Sir John said Reform's tally was down on the 30% figure which the BBC published after last year's local elections.
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