UKIP 'double job' MEP Nathan Gill faces party expulsion
UKIP's leader in Wales faces expulsion from the party because of concerns over him working as both an MEP and assembly member.
The party's National Executive Committee voted to remove Nathan Gill unless he resigns from one of his elected positions.
Mr Gill has represented Wales in the European Parliament since 2014 and won a seat in the Senedd in May's election.
The MEP said the decision was "anti-democratic" and "baseless".
In an open letter, Mr Gill said: "It is not for the NEC to inform me how to fulfil the mandate that I have been given by the electorate."
He said he was "not going to be coerced by the NEC into resigning positions to which I have been either elected by the people of Wales or appointed by the party leader".
Mr Gill added the decision may be "in clear breach of contract".
A group meeting of UKIP's AMs is expected to discuss the issue on Tuesday.
UKIP Assembly group leader Neil Hamilton, speaking at the National Eisteddfod, said the vote on Mr Gill "had nothing to do with infighting".
"When he was seeking nomination for the assembly he said that if he was elected he would stand down as MEP and devote his time to furthering the interests of the people of Wales. He hasn't done so," he said.
"I'm afraid we're only seven out of 60 members of the assembly. We can't afford part-time or half-time AMs. He has a choice. You can't do two jobs properly. He has to choose one or the other."
On Friday, the chairman of UKIP Wales, Chris Smart, said Mr Gill should step down as an MEP as a successor was now available.
Mr Smart said James Cole - who was number two on UKIP's Wales list at the 2014 European election and who has since rejoined the party - was ready to take over.
However, Mr Gill previously said calls for him to quit the role had been made out of "malice".