The Welsh Assembly member, who defected from the Conservatives to Ukip in 2014, will now sit with the Conservative group in the Senedd following his resignation from Ukip.
But Tory party sources have stressed Mr Reckless, whose initial defection to Ukip sparked a bitter reaction from then prime minister David Cameron, has not rejoined as a Conservative Party member.
It has been reported Mr Reckless will be blocked from formally becoming a Tory party member.
Neil Hamilton, Ukip's leader in the Welsh Assembly, mocked Mr Reckless for having “burned his boats” with both his party and the Tories.
He told Express.co.uk: "He's joined the Conservative group but the Conservative Party don't want him.
"He's betrayed Ukip members, he's betrayed Ukip voters who elected him and he's betrayed the trust of everyone who believes in high standards of politics.”
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Mr Hamilton, a former Tory MP himself, also repeated Ukip’s call for Mr Reckless to resign from the Senedd as he stood for election as one of their candidates in the Welsh Assembly’s regional list system
He said: "He should resign his seat as the people elected a Ukip Assembly Member, they didn't elect Mark Reckless.”
Mr Hamilton predicted it was now “curtains” for Mr Reckless’s political career and suggested he would not be allowed to stand as a Tory candidate at the next Welsh Assembly elections.
He said: "He can see out his term in the Assembly but then it's curtains. The Tories have no incentive to allow him to rejoin the party.
"He's been used by [leader of Welsh Conservatives] Andrew RT Davies for internal reasons within the Assembly. They don't want him as a candidate.”
He should resign his seat as the people elected a Ukip Assembly Member, they didn't elect Mark Reckless.
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Neil Hamilton, Ukip's leader in the Welsh Assembly
Mr Reckless’s defection now makes the Conservatives the largest opposition group in the Welsh Assembly, boosting Mr Davies’ status in the Senedd.
Mr Hamilton suggested the Tories may have leaked news of Mr Reckless’s possible defection from Ukip this week to leave him “damned if he does and damned if he doesn't”.
He said: “He would have made his move to the Conservatives on the basis he was allowed to be a member of the Tory Party but they told him he can’t.”
Mr Hamilton claimed this meant the Tories had now been able to secure a boost in their Welsh Assembly status but have not had to give Mr Reckless ”anything worthwhile” in return.
He added: “He’s finished. He’s shown he can't be trusted. He ratted on the Tories and now he's ratted on Ukip. I’m not sure there’s anyone else left he can rat on.”
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Mr Hamilton insisted the departure of Mr Reckless was “not a blow for Ukip for Wales” but would see the party’s allowances cut in the Welsh Assembly.
He said: “Apart from that it's not going to make any difference. We’re all just going to have to work a bit harder.”
Mr Reckless left the Conservatives to join Ukip in September 2014, following the path of fellow Tory defector Douglas Carswell.
He called and won a by-election as a Ukip candidate in his Rochester & Strood consituency in November 2014, but was later booted out of Parliament by voters at the May 2015 general election.
He was then elected to the Welsh Assembly on the regional list system in May last year.