UKIP now has just one member of the Welsh Assembly, after its former group leader Gareth Bennett quit the party.
The party saw seven members elected to the Senedd in 2016, but ex-Tory MP Neil Hamilton is now the last one left.
Mr Bennett will sit as an independent and back Boris Johnson's Brexit deal, claiming the UK will not leave the EU if the Tories lose the election.
A UKIP spokesman said: "Members have come and gone and this is no different."
UKIP has struggled since helping to secure a leave vote in the 2016 EU referendum, a few weeks after the party won its first assembly seats.
After coming top across the UK in the 2014 European elections, UKIP finished eighth in the polls in May, losing all its MEPs as the Brexit Party - led by former UKIP leader Nigel Farage - swept to victory.
Both UKIP and the Brexit Party claim the agreement the prime minister made in October with EU leaders is not a proper Brexit because it will still leave the UK bound by European rules.
At a UKIP conference in Newport in September, which was overshadowed by newly-elected party leader Richard Braine refusing to attend, Mr Bennett admitted the assembly group tended to "specialise in having bust-ups with each other".
In a statement on Thursday, Mr Bennett, a South Wales Central regional AM and former UKIP Senedd group leader, said: "The British public, who voted to leave the European Union, have waited three-and-a-half years - and we still haven't left.
"There is now a grave danger that, if Boris Johnson's deal doesn't go through, then we will not leave at all.
"Those of us who wanted a harder deal now have to realise that Boris's deal is the best we can get at this time."
Accusing the Brexit Party and UKIP of "effectively trying to sabotage Brexit" by opposing the deal, Mr Bennett said he had made the decision to resign from UKIP with immediate effect, in order to sit as an independent AM.
Four former UKIP AMs - Mark Reckless, Mandy Jones, Caroline Jones and David Rowlands - joined Mr Farage's Brexit Party in May, with three of them having already left UKIP to sit as independents.
Michelle Brown quit the UKIP group in March and still sits as an independent.
Mr Bennett has been no stranger to controversy since he was elected.
He was suspended from the Senedd for a week without pay earlier this year after superimposing the head of a female Labour assembly member onto the image of a barmaid in a video he made.
Mr Bennett was also officially censured last December for spending almost £10,000 on an office that never opened.
A UKIP spokesman said it was "regrettable" that Mr Bennett had decided to leave the party.
However, he added: "UKIP has been campaigning for Brexit for 27 years. Members have come and gone and this is no different.
"The party will continue to fight for a clean break from the European Union, the unequivocal decision made by 17.4m people who voted leave in June 2016."
A spokesman for the Brexit Party group in the assembly said it would not be commenting on Mr Bennett's decision.