Nathan Gill has confirmed he will stand for Nigel Farage's new Brexit Party if the UK's departure from the European Union is delayed.
The UK is due to leave on 29 March but there have been calls to delay it in order to secure an acceptable plan for avoiding a hard border in Ireland.
Mr Gill is currently an independent MEP for Wales and the European Parliament election takes place on 23 May.
The Brexit Party was recognised by the Electoral Commission on Friday.
It has been set up in order to fight the election if Brexit is delayed.
Theresa May's Brexit deal - which she spent months negotiating and had agreed with the EU - covers the terms of the UK's divorce and the framework of future relations.
But it was rejected by the UK Parliament and if it is not approved when the time comes to leave on 29 March, the default position would be a no-deal Brexit. But some MPs have called for a delay.
Mr Gill, from Anglesey, said delaying Brexit would be "unacceptable" and there was a 60% chance European elections would be held in the UK.
"The Brexit Party will hoover up all of the discontent that is out there in the country and it's necessary to allow Labour and the Conservatives to understand that if you betray us, then you will suffer at the electoral box," he told BBC Radio Wales' Sunday Supplement programme.
"I know Europe does not want it. They just want to get on with their project and they certainly don't want a Nigel Farage and 45 other Nigel Farages coming back into the heart of the European parliament.
"That will be a big block and with Nigel as a ringleader it would be a huge sway within the European Parliament itself.
"They pretty much don't want him to come back, we don't have to want to go back but if we are betrayed then we will."
He said the need for a new party was because UKIP, which he quit in December, had "gone down the wrong path".