Nigel Farage says he would stand as a candidate for a new Brexit party if the UK's departure from the EU is delayed.
Britain is set to leave the bloc on March 29.
But if London delays its exit by more than three months it will have to take part in May's elections for the European Parliament.
Farage, ex-leader of the United Kingdom Independence Party (UKIP), said he would take part in the poll with the newly-formed Brexit Party.
The movement, formally recognised by the Electoral Commission, is lead by former UKIP candidate Catherine Blaiklock.
Farage, in an opinion piece for The Daily Telegraphnewspaper, wrote: "The party was founded with my full support and with the intention of fighting the European elections on May 23 if Brexit has not been delivered by then."
With the UK set to leave the EU in 49 days, no agreement with the bloc has been reached since MPs voted down a deal in November.
Brussels and London are currently trying to amend the agreement but the possibility of a last-minute no-deal or a delay appear increasingly likely.
"I have made it clear many times that I will not stand by and do nothing if the referendum result is betrayed, so should this election need to be contested, I will stand as a candidate for the Brexit Party and I will give it my all," Farage said.
The Telegraph reported thousands of Conservative members are "likely" to defect to the party.
Farage said in a tweet that he hoped the Conservative and Labour parties recognised the Brexit Party "as a threat".