Four former UKIP AMs have joined Nigel Farage's Brexit Party, announcing plans to form an assembly group.
Mr Farage declared Mark Reckless group leader on a visit to Cardiff on Wednesday.
The planned group, which includes Mandy Jones, Caroline Jones and David Rowlands, is subject to assembly approval. Some Plaid Cymru and Labour AMs want it stopped.
Mr Farage said: "The leavers in Wales are all coming back together again."
"I'm very pleased as leader of the Brexit Party to welcome four members of the Welsh Assembly who will now re-designate, as of this afternoon, as Brexit Party members," he added.
Mr Farage, who also held a rally in Merthyr Tydfil on Wednesday, launched the party last month for the European elections on 23 May.
UKIP confirmed Mr Rowlands had resigned from the party - meaning it no longer has a Senedd group and will have to lose staff.
Mr Reckless said: "Brexit is being blocked."
The new group is "supporting the Brexit Party in everything that they are doing to protect our democracy and to ensure that Brexit is delivered", he added.
Mr Reckless had been a UKIP AM from 2016 to 2017, when he joined the Tory group. He left it on Tuesday ahead of the announcement.
He told Gareth Lewis on BBC Radio Wales he had believed Prime Minister Theresa May when she said "we were going to leave the EU on the 29 March this year, with or without a deal".
"Unfortunately she's broken her word," he said.
Former UKIP assembly leader Neil Hamilton called for all four to resign. Mr Reckless said he would "love" to hold a by-election but was unable to do so under the system he was elected under.
The announcement follows years of turmoil in UKIP - it entered the assembly with seven AMs, but now just has two.
Mr Farage, who had criticised Mark Reckless when he left UKIP, admitted to "one or two cross words with all of them" over the years. "That's all forgotten now", he said.
He said he had "hand-picked" who he wanted to join the Brexit Party. "There is a renewed sense of purpose and unity amongst these people," he said
The party leader, who appointed Mr Reckless, promised the group would have its own "radical" policies - the party does not yet have any, beyond wanting to leave the EU.
'Frustration and anger' at rally
By James Williams, BBC Wales Brexit correspondent
Despite the best efforts of anti-Brexit protestors who blocked the road, about 600 or so people decided to spend a beautiful sunny evening at a political event in a car park in Merthyr Tydfil.
It was a smaller and more subdued gathering than the Brexit Party's rally in Newport a fortnight ago.
But what was remarkable was that, for the vast majority of people I spoke to, this was their first time at a political gathering.
Frustration and anger with the Brexit delay had driven them to attend.
Some were Labour voters, most were frustrated Tories, but they were all united in their belief that the two-party system is a busted flush.
At the very least, the people here tonight are betting on the Brexit Party to fix things.
David Rowlands told BBC Wales he had become "increasingly disappointed" with the way that UKIP was heading. He had disagreed abolishing the assembly - a key policy of UKIP assembly leader Gareth Bennett.
The defining moment, he said, was the "interference" by UKIP HQ in the Newport West by-election, deciding to "displace the candidate that the South Wales East region had selected", Stan Edwards, and replace him with AM Neil Hamilton.
Mr Rowlands said it was a "particular slight" that he was not asked to stand.
"David was so annoyed about interference in candidate selection that he joined a party whose candidates are all selected by one man," Mr Bennett said in response.
The group of four wrote to the presiding officer Elin Jones to request they form a Brexit Party group "with immediate effect", with Mr Reckless as leader and Mr Rowlands resigning from UKIP.
An assembly commission spokeswoman confirmed that the request was being considered by Ms Jones.
Some Labour and Plaid Cymru AMs are considering trying to amend the rules, known as standing orders, to prevent groups representing parties who did not stand in an assembly election from being recognised.
Alun Davies, Blaenau Gwent Labour AM, told the Senedd there were AMs on both sides of the chamber who were "deeply disturbed at the undermining of our democracy by events today".
"These chancers did not stand for election under any party label they are using today," he said. "They are using this in order to access public resources and public money without standing for election."
He asked the presiding officer to use "some discretion" to consider the matter over a "period of time" to allow AMs to consider their own thoughts and responses.
By BBC Wales political editor Felicity Evans
This is a major coup for Nigel Farage and his new Brexit Party, just over a week before polling day in the European Parliament elections.
Assuming existing assembly rules are followed, the Brexit Party will have achieved recognition and representation in one of the UK's parliaments without even facing an election.
While this is a boost for Mr Farage's new party, it's a disaster for his old one. The defection of David Rowlands has torpedoed UKIP.
Wales was UKIP's last bastion in terms of significant electoral representation. They're now left with two AMs and a loss of resources and status within the Welsh Assembly.
Today's announcement raises the stakes for Welsh Labour too - at stake is a century of Labour dominance in Welsh elections.
Its new leader, Mark Drakeford, will not want to see another party come out on top in his first electoral test.
A Plaid Cymru spokesman said the Brexit Party group had "no democratic mandate and they should not be permitted to form a group nor given access to public resources and funding in the Senedd".
"This is a group which stands for nothing more than its own short-term, narrow self-interest," the Labour group said.
Mr Bennett, the UKIP AM who led the party in the assembly when it was a group, said Mr Rowlands departure from UKIP will "inevitably lead to job losses for our hard working support staff, something which David was well aware of before reaching his decision".
"I am sorry and disappointed that David has decided to take this step," he said.
Who are the group?
- South Wales East AM Mark Reckless is a former Tory MP who defected to UKIP in 2014. After losing his seat in 2015 he was elected to the assembly a year later but later defected to the Conservative group
- Caroline Jones, who represents South Wales West, was briefly UKIP assembly leader during 2018. She left the party last year
- Mandy Jones is the only one of the four who was not elected in 2016 - replacing Nathan Gill as a North Wales AM when he resigned. She was never allowed to join the UKIP team in the assembly but was previously a party member. She has sat as an independent
- David Rowlands, who also represents South Wales East, resigned a UKIP AM on Wednesday. He had been in the party since the 1990s
The European Parliament elections in Wales
There are eight parties fighting for four Welsh seats in the European elections on 23 May.
Welsh Labour, the Welsh Conservatives, Plaid Cymru, the Welsh Liberal Democrats, UKIP and the Green Party are joined by Change UK and the Brexit Party.