Former UKIP AM Mark Reckless has left the Conservative group in the assembly, he has announced.
He said he has left the party group because the UK government “has broken its promise to deliver Brexit”. Mr Reckless had not been a member of the party.
It comes as speculation mounts a Brexit Party assembly group could be formed.
Tory group leader Paul Davies thanked him “for his valuable contribution” over the last two years.
Mr Davies said: “I appreciate there are strong views on Brexit but our commitment is clear – Wales and the rest of the United Kingdom must leave the European Union.”
A spokesman for the South Wales East independent AM, who was not conducting interviews on Tuesday, said he had left the Tory group on “good terms”.
Nigel Farage, Brexit Party leader, is due to visit Merthyr Tydfil on Wednesday.
Speculation in recent weeks has suggested a new group could be formed in the assembly including some of the Brexiteer AMs in the assembly.
Caroline Jones is known to have signed up to the Brexit Party as a supporter and was at a Newport rally earlier in May, as was Mr Reckless.
However, there has been no confirmation from any of the individuals – Ms Jones, Mark Reckless, David Rowlands and Mandy Jones – that are thought could be involved.
One source said Mr Reckless’ resignation was a step towards a new group, but did not mean it was a “done deal”.
Mr Rowlands, if he was to defect, would leave UKIP with two AMs, not enough to form a party group which grants AMs more resources and staff.
A UKIP group source said on Monday they had no concerns about Mr Rowlands, however, while a Brexit Party spokesman said a newspaper report of discussions over a new group was a “great work of speculation”.
It is the third time Mr Reckless has defected from a political group.
Mr Reckless joined the Welsh Conservative group in 2017 after leaving UKIP, but did not rejoin the party he had served as an MP.
‘At face value’
UKIP AM Neil Hamilton said David Rowlands told him and UKIP leader Gerard Batten that he would not be defecting.
Mr Hamilton: “He told me and he told Gerard Batten that he’s not going to defect.
“I take his word at face value.”
Of the reports and speculation of a Brexit group in the assembly, Mr Hamilton said: “Given the history of the selfishness of those who were elected in 2016 as UKIP AMs to go off and do their own thing, out of frustrated ambition or whatever reason, of course I’m concerned when people betrayed the trust of the party that gave them the opportunity to become AMs and also the people who voted for them in the election.
“I don’t think that’s a very honest basis to form a new party.”