UKIP Chairman Paul Oakden said, "We are disappointed to hear that Mark Reckless has decided to leave UKIP in order to join the Conservatives in the Welsh Assembly.
"When Mark joined UKIP in 2014 we admired his long-term commitment to Brexit and his decision to put his country first.
"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. In the campaign that followed, our membership worked incredibly hard to ensure that Mark retained his seat.
"After losing his seat at the General Election, Mark was a popular colleague and a diligent Head of Policy for the Party, before being awarded a top position in the UKIP list for the Welsh Assembly Elections in 2016. Those elections gave voters in Wales the opportunity to support a party rather than a particular candidate and one in eight voters backed UKIP. As a result, Mark was one of two UKIP candidates from South Wales East elected to the Senedd.
"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."