British Prime Minister Rishi Sunak warned on Friday that a vote for Nigel Farage's Reform UK Party would hand electoral victory to Labour, after a poll put the right-wing group ahead of Sunak's Conservatives for the first time.

Farage is one of Britain's most recognisable politicians best known for his decades long - and eventually successful - crusade for Britain to leave the European Union.

His Reform UK party began life as the Brexit Party in 2018, during the tortuous negotiations on how Britain would leave the bloc.

Farage's unexpected return to frontline politics has split support among right-of-centre voters, badly damaging Sunak's hopes of winning an election he was already predicted to lose to the opposition Labour Party.

That impact was shown in a YouGov poll on Thursday which put Reform UK on 19 percent, up from 17 percent previously, and the Conservative Party unchanged on 18 percent.

Labour, led by Keir Starmer, topped the poll with 37 percent.

"If this poll was replicated it would hand a blank cheque to Labour," Sunak said in Italy, where he is attending the G7 summit, according to remarks reported by British media.

"Ultimately a vote for anyone who is not a Conservative candidate makes it more likely that Keir Starmer is in No.10."

Farage said his party had made a "phenomenal" start to the campaign and was now the real opposition to Starmer's Labour. (Reuters)