Nigel Farage has accused the Conservatives of copying the rhetoric of Reform UK, "but not the actions".
Mr Farage – who founded Reform from the ashes of the Brexit Party – was the star turn at its annual conference.
He said the Tories had become a "social democrat party in all but name" with "big-state, high-tax" policies.
Mr Farage and current Reform UK leader Richard Tice both argue the government has failed to properly deliver Brexit or to control immigration.
Mr Farage also used his speech to Reform conference in central London to repeat his insistence that he would not rejoin the Conservative Party under its current leadership.
His appearance at the Tory conference in Manchester – the first time he had attended such an event in decades – and the welcome he received from many senior Conservatives prompted speculation that he might consider returning to the Tory fold.
In Manchester, he ruled out joining the party "as it currently is", but added: "Never say never."
Asked about Mr Farage earlier this week, Rishi Sunak told GB News: "Look, the Tory party is a broad church. I welcome lots of people who want to subscribe to our ideals, to our values."
At Reform UK's conference in London, Mr Farage responded by saying: "It's very, very sweet of you, prime minister, but I'm really sorry, the answer is no, I will not."
He said he would focus his efforts on backing Reform UK, saying there was a "gap in the political market" for the party to fill, although he has ruled out standing as a candidate for the party at the next election.
Comparing its position to that of UKIP in 2012, Mr Farage said: "This party has been bubbling away quietly just under the radar."
Reform UK has never had any MPs. It performed poorly in May's local elections, failing to gain any seats despite fielding nearly 500 candidates, and losing half its councillors, retaining eight.
But the party -which campaigns for zero net migration, tax cuts for those on lower incomes and against net zero climate targets – has consistently been in fourth place in opinion polls at around 6%, just ahead of the Green Party.
It restored the word, "Brexit", to its party logo for its conference, saying it intended to reclaim the word from the Conservatives in the run-up to a general election expected next year.
In his speech to the conference, Mr Tice set out his Reform's plan to end illegal immigration.
Appearing to refer to people who arrive in the UK having crossed the Channel in small boats, he said: "Let's pick up, let's take back to France, and then show the EU leaders this is what they need to do in the Mediterranean.
"Only then will this crisis, this hurricane of migration that Suella Braverman talked about, only then will it be stopped."
Mr Tice was alluding to the home secretary's speech at the Tory conference, in which she said a "hurricane" of mass migration was coming, causing unease among some senior Conservatives.
He said the government offered "warm words", but had no "idea how to stop the boats".
"We are mass immigration Britain. The numbers are so big, it's hard to calculate," said Mr Tice.
Saying the Conservatives had "broken Britain", Mr Tice also turned his fire on Labour, arguing it would "bankrupt Britain".
Reform UK would scrap net-zero targets, tackle immigration and cut NHS waiting lists, he said. The party's conference carried the slogan, Let's Save Britain.
The party plans to field 630 candidates across England, Scotland and Wales in the next general election.
It has ruled out standing aside to allow the Conservatives to gain more seats, as it did in more than 300 seats under its Brexit Party name in 2019 to avoid splitting the Leave vote.
Tories imitating our policies, says Reform UK leader
Farage leaves door open to re-joining Tories
Reform UK struggles in local elections
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