Reform UK would cost the Tories 39 seats if it wins 10 percent of the vote at a general election, according to new analysis.
The research by the More in Common think tank suggests Labour would gain 32 constituencies while the Lib Dems would take five.
If the Nigel Farage-linked party hit 12 percent of votes, the Conservatives would lose 47 seats.
And if Reform UK, formerly the Brexit Party, won 15 percent it would cost the Tories 63 seats.
The insurgent party led by Richard Tice is expected to struggle to win seats outright but would split the right-wing vote.
More in Common director Luke Tryl told the Sunday Times: “While you can’t simply add up the Conservative and Reform vote and assume it would give a combined vote share if the two parties joined forces, we do know that for every ten votes the party gets, they take six of those from the Conservatives and just half a vote from Labour.
“Even though Reform won’t win seats themselves, for every ten votes the party gets Labour effectively gain 5.5 over the Conservatives.”
Reform UK secured 13 percent of the vote in the Wellingborough by-election and 10.4 percent in Kingswood last Thursday, with the two previously safe Tory seats going to Labour.
Mr Farage, who is Reform’s honorary president, hailed it as a “coming-of-age moment”.
Meanwhile, Mr Tice said the results showed the party had overtaken the Lib Dems as the “third-largest political force in the UK”.
He has insisted he will not strike any electoral pacts with the Conservatives.
It comes amid ongoing speculation over whether Mr Farage will make a sensational political comeback.
The former Ukip leader, who came third on ITV reality show I’m A Celebrity… Get Me Out Of Here, has refused to rule out a return to frontline politics.
