UKIP AM Mark Reckless is set to quit the party and vote with the Conservative group in the Assembly.
Tory sources have said Mr Reckless, the AM for South Wales East, has held talks with Andrew RT Davies, leader of the Welsh Conservatives.
An announcement is due on Thursday which would see Mr Reckless join the Tory group in Cardiff Bay.
It is understood Mr Reckless will be an independent AM rather than re-join the Conservative Party.
There is resistance to re-admitting Mr Reckless following his dramatic defection from the Conservatives to join UKIP in September 2014.
The then-MP fought and won a by-election in his Rochester and Strood seat in November that year, but lost his seat in the May 2015 General Election.
He was elected to the Assembly in May 2016 as UKIP won their first seats in Cardiff Bay, and he is currently chairman of the Climate Change, Environment and Rural Affairs Committee.
He had also worked as a researcher for MP Douglas Carswell, who left UKIP last month.
The move would mean the Conservative group would become the second-largest, with 12 seats to Plaid Cymru’s 11.