Former MP Mark Reckless has quit Ukip to rejoin the Conservatives, the party has announced.
Mr Reckless, who now sits in the Welsh Assembly, praised the "exemplary" leadership of Theresa May since becoming Prime Minister.
"She has been steadfast in her position to deliver on the wishes of the people of Wales and the United Kingdom, building a united country that works for everyone and not just the privileged few," he said.
Mr Reckless's decision to rejoin the Conservatives means they overtake Plaid Cymru as the second largest party in the Assembly, making them the official opposition to Labour.
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."
The re-defection of Mr Reckless is a further blow for Ukip after its only MP Douglas Carswell - another defector from the Tories - announced he was leaving to sit as a Conservative.
Mr Reckless originally joined Ukip in 2014, but after winning a by-election in his Rochester and Strood seat, he was voted out of the Commons in the 2015 general election.
He was subsequently elected to the Welsh Assembly on a Ukip ticket in May 2016.
Ukip chairman Paul Oakden said they were "disappointed" at Mr Reckless's decision.
He said, having been elected to assembly through the party list system, Mr Reckless should now give up his seat for a Ukip candidate.
"Having been elected as a Conservative in Rochester & Strood, Mark didn't think it right to sit as an MP for another party without asking the electorate for their endorsement in a by-election. A position that was right as it was honourable," he said.
"In the campaign that followed, our membership worked incredibly hard to ensure that Mark retained his seat.
"It is now incumbent on Mark Reckless to relinquish a position he has only by virtue of a Ukip mandate. The position should go to the next Ukip candidate on the regional list."
A spokesman for the Ukip group in the Welsh Assembly said: "This defection is a slap in the face for the voters of South East Wales who voted for Ukip in 2016 in the good faith they were electing a Ukip Assembly Member.
"If Mark Reckless has any sense of honour he will resign his seat without delay and stand by the commitment he gave in 2014 when he defected from the Tories - namely not to switch parties without the consent of his constituents.
"As was the case with Nathan Gill, Ukip were elected through a proportional list system. Neither Mr Gill nor Mr Reckless have a democratic mandate to remain in the Assembly. Both should resign and allow Ukip to nominate local members to take their place.
"As we commence Brexit negotiations, the people of Wales need to know now, more than ever, that their interests will be protected.
"Ukip is the only party in the Welsh Assembly committed to a complete withdrawal from the European Union. It is therefore vital that there is a strong Ukip voice in the Assembly.
"Ukip received four million votes in the last general election - more than the SNP and Liberal Democrats combined. In a few months this will be history and we will go on representing the hard-working people of Wales, with or without Mark Reckless in the group."