Tory AMs have put their positions “in jeopardy” by allowing Mark Reckless to join them, a UK Conservatives source has told BBC Wales.
Mr Reckless quit UKIP in Cardiff Bay on Thursday to sit as a Tory AM.
Conservative Party rules state that only party members can be Tory AMs.
Welsh Conservative leader Andrew RT Davies denied the party constitution had been suspended to allow Mr Reckless to join his group in the assembly.
Mr Reckless defected from the Conservatives as an MP for Rochester and Strood in 2014. He became the UKIP AM for South Wales East in May 2016.
A second Conservative source told BBC Wales: “The entire Welsh Conservative group is facing de-selection.”
Senior Tories, including Welsh Secretary Alun Cairns, have insisted there are no plans to allow Mr Reckless to re-join the party.
Mr Reckless’ defection back to the Conservatives mean the Tories take over as the official opposition to the Labour government in the Welsh Assembly.
The UK Conservative source told BBC Wales: “This is viewed very seriously indeed in London.
“Every member of that group who voted to suspend the constitution has put their position in jeopardy.
“Do not imagine for a moment that the party in London will roll over on this.”
The source also said many rank and file Conservatives were still furious at Mr Reckless for defecting in such a high-profile way in 2014, on the eve of a pre-election party conference and had campaigned against him in a resulting by-election.
A Conservative source said that, in order to allow Mr Reckless to become a Tory AM, the party’s other AMs voted to suspend their group’s rules.
However Andrew RT Davies, Welsh Conservative leader, denied that there had been a vote to suspend the party constitution and told BBC Radio Wales it would not be in his AMs’ “gift” to do so.
Following criticism from Conservative sources, he said: “I am always a little sceptical when it’s sources.”
Mr Davies added: “I accept some MPs, because of historical arrangements and historical actions by Mark, feel aggrieved by Mark’s actions, but he has not applied to join the party.”
He insisted there was “unanimous agreement” among his group “that Mark was a fit and proper person to vote with and work with the Conservative group in the assembly”.
“There is little or no need to get excited about this,” he added.
Craig Williams, Cardiff North MP, became the latest Welsh Conservative MP to criticise the decision when he spoke to BBC Radio Wales on Friday morning.
Mr Williams demanded answers as to how the arrangement might work, emphasising Mr Reckless was not a Tory party member and insisted there should have been consultation with party activists.
Mr Williams told the Good Morning Wales programme: “I don’t want to be knocking on doors with Mark Reckless in Cardiff North, and he won’t be invited.”
He said the party’s constitution was not “optional” and the rules could not be bent.
However, two senior grassroots Tories in South Wales East told BBC Wales they are happy with the decision to bring Mark Reckless into the group.
One welcomed the chance for the Conservatives to become the main opposition, saying: “People make mistakes in their political lives and I think we should give him a chance.”
Yesterday the Welsh Conservative Party press release warmly welcomed Mark Reckless into their team in the assembly, as a hard-working member.
This morning, after the fallout from others within the party, Andrew RT Davies was using slightly more measured language; that Mr Reckless would “vote with and work with the Conservative group in the assembly”.
That is not quite the same thing – so what’s going on?
MPs have been lining up to criticise the decision of the assembly group to welcome Mr Reckless into their ranks. They claim to have the backing of Number 10, so it seems we have a bit of a stalemate.
I’m told that Conservative HQ officials are happy to bide their time on this, but are totally unwilling for Mr Reckless to be an AM for the party.
Andrew RT Davies isn’t known to be a person who backs down from a challenge and his relationship with party HQ has been strained over the past year or so.