Tory defector Mark Reckless has quit Ukip and joined the Conservatives in the Welsh Assembly.
Mr Reckless defected to Ukip from the Tories in 2014, winning a subsequent Parliamentary by-election but losing his Rochester and Strood seat in the 2015 general election. He has since been elected as a regional Assembly Member in South Wales East.
The move is a fresh blow to Ukip after its only MP Douglas Carswell - another defector from the Tories - announced he was leaving to sit as an independent.
Ukip Chairman Paul Oakden said Mr Reckless should now relinquish his position gained "by virtue of a UKIP mandate".
Mr Reckless said it was "job done" now that Article 50, sparking Britain's departure from the EU, had been triggered.
"I leave Ukip positively, having achieved our joint aim, a successful referendum to leave the EU," he said.
"I campaigned all my adult life for us to leave the EU. Now we are, under a Conservative government. I support Theresa May, and her team of Brexit ministers, to deliver."
Mr Reckless said his defection would help the Conservatives provide a strong opposition to Labour in Wales, with his switch meaning the Tories overtake Plaid Cymru as the second largest party in the Assembly.
The move was warmly welcomed by the leader of the Conservative group in the Assembly, Andrew RT Davies.
"Since being elected to the Senedd last year, Mark has proven himself to be a hard-working and dedicated AM who has been an effective representative for the South East Wales region," he said.
"He will now be able to continue this work as part of a strong and united team which will be the official opposition in the Assembly.
"Mark's decision to join the group is testament to the strength and unity of the Welsh Conservative team in Cardiff Bay which provides the only real opposition to a tired and complacent Labour administration."
Mr Oakden said the party was "disappointed" by his defection.
"It is now incumbent on Mark Reckless to relinquish a position he has only by virtue of a UKIP mandate," he said. "The position should go to the next UKIP candidate on the regional list."