UKIP AM Mark Reckless has confirmed he is joining the Conservative group in the Welsh Assembly.
The South Wales East AM will sit as a Tory but will not join the party.
Conservative group leader Andrew RT Davies welcomed a "hard-working and dedicated" AM, saying it made his party the official opposition to Labour.
But Gower MP and ex-AM Byron Davies said allowing Mr Reckless into the group was "not a particularly bright idea".
When questioned about Andrew RT Davies' leadership of the Tory group on BBC Radio Wales, the Gower MP said "we have to live with that".
With regard to how the Tory group should work with Mr Reckless, he said: "I would be very, very cautious about how they deal with him."
Welsh Secretary Alun Cairns said: "Mark Reckless is not a member of the Conservative Party. There are absolutely no plans for him to become a member of the Conservative Party."
A Conservative Party spokesman said: "Decisions about who sits with the Conservative group in the Welsh Assembly are a matter for the group in the Welsh Assembly."
In a statement, Mr Reckless said he had been "thoroughly impressed by the performance and discipline of Andrew RT Davies and the Welsh Conservative group".
He said Theresa May's leadership as Prime Minister had been "exemplary" and that she had been "steadfast in her position to deliver on the wishes of the people of Wales and the United Kingdom" in relation to Brexit.
Assembly Presiding Officer Elin Jones said the former UKIP AM could be a member of the Conservative group.
Andrew RT Davies said Mr Reckless had "proven himself to be a hard-working and dedicated AM who has been an effective representative for the South East Wales region".
"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," he added.
Mr Reckless told BBC Wales he knew there was still "bad blood" from the time he quit the Tories to join UKIP.
"It's not for me to waltz back into the party with any sense of entitlement," he said.
"I want to focus my efforts here with some humility."
UKIP assembly group leader Neil Hamilton accused his former UKIP colleague of showing "weakness of character".
"He didn't have the courtesy or the courage to speak to me about any doubts he had about his future in UKIP or what he might get from the Conservative Party," he told BBC Wales.
"Fundamentally he was elected to the assembly not as Mark Reckless but as a UKIP candidate for the South East Wales region.
"He's betrayed the trust of all of those who selected him in the first place to be a candidate and all of those who worked to get him elected to the assembly.
"He's got no mandate to sit in the assembly as a member of the Conservative group."
UKIP chairman Paul Oakden said it was "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," he said.
Mr Reckless was Conservative MP for Rochester and Strood when he defected to UKIP in 2014.
He voluntarily quit the seat to fight and win it in a by-election for UKIP, but lost it at the 2015 general election.
There is no requirement under assembly rules for either regional or constituency AMs to stand down when they leave the party they were elected to represent.
Mr Reckless told BBC Wales he would "love to be able to put my decision to the electorate" as he did in 2014, but said assembly rules regarding members elected via a regional list prevented this.
Plaid Cymru leader Leanne Wood said the changing arithmetic in the Senedd - with Plaid now the third-biggest party - would mean "very little in actual fact".
She added: "We've got a strong team of assembly members all of whom are working very hard for the constituencies they represent, and for Plaid Cymru as a whole.
"We are putting the Welsh national interest at the top of the agenda at every opportunity. We might be a smaller team but we certainly are a very effective team."
Analysis by BBC Wales political correspondent Tomos Livingstone
Normally the defection of a politician from one party to another prompts a stream of abuse from the member's former home and the sound of champagne corks popping at their new abode.
There's been plenty of the former from UKIP now that Mark Reckless has left, but not everyone in the Conservative Party is celebrating today's development.
Mr Reckless made the opposite journey in 2014, and anger at that decision is still plain for all to see.
That's why the South Wales East AM is going to be a Conservative AM, but not, rather oddly, a member of the Conservative Party.
Has Andrew RT Davies therefore performed a coup, defying the wishes of Welsh Conservative MPs who think he's mis-read the party mood, and making his group the second-largest in the Senedd?
Or has he needlessly made some powerful enemies who might want to re-visit the whole affair in the weeks and months ahead?
This could be the beginning of the story rather than the end.