Nigel Farage hailed sweeping election wins for Reform UK as a “historic shift in British politics” on a day when the populist party made gains at the expense of Labour and the Conservatives.

Reform made advances in heartland areas of both parties, clocking up substantial early results in the English local elections by taking control of Essex county council, Havering – its first London local authority – and Sunderland city council.

However, the results were not without setbacks, for example in Harlow, a past general election bellwether, while one prominent pollster suggested the party may have peaked and that Farage would have reason to be “privately worried”.

Nevertheless, Reform established beachheads for the next general election in areas including Essex, home to the seats of prominent Tory MPs including Kemi Badenoch and James Cleverly, where Farage’s party went from having a single representative to taking control of the council.

“It’s a big, big day, not just for our party but for a complete reshaping of British politics in every way,” Farage said as he appeared on Friday outside Havering town hall, in a borough on the eastern border of Greater London where many voters identify more closely with neighbouring Essex.

Farage said the party was “two-thirds” of the way to where it wanted to be for the general election when it came to planning and fundraising. The door was now closed to Tory defectors, he said, but “the time is now” for conversations with “patriotic old Labour” MPs.

Reform’s first major gain was Essex, a key target for Farage’s party and a county council where the Tories had enjoyed majority control since 2001. Fifty-three Reform councillors were elected, a smaller majority than some had tipped, and will face a 24-strong opposition that includes 13 Tories.