Peter Robinson has been accused of using Ian Paisley to sell a deal on power-sharing to the DUP faithful – and then ditching him when it was done.
The allegation comes from TUV leader Jim Allister, a former DUP MLA, who said there had been tension between both men for many years.
Mr Allister, who quit the DUP in 2007 in protest at its power-sharing deal with Sinn Fein, said he had no doubt Mr Paisley was pushed.
The DUP refused to respond to Mr Allister's comments.
However, the party has repeatedly rejected allegations that Mr Paisley was ousted, adding that the timing of his departure was entirely a matter for him.
Mr Allister claimed only Mr Paisley could persuade the DUP to enter power with Sinn Fein.
"Peter Robinson could never have sold the deal to the party faithful, but Paisley could and he did," he said.
"For all their divergence over the years there came a point at St Andrews where they had common cause – Paisley wanted power for power's sake, Robinson wanted the deal also, but was cute enough to recognise that Ian Paisley could sell it. Once he had done that, his work was done."
Mr Allister said the timing of Mr Paisley's departure was crucial.
"They were careful not to ditch him before they had sold the package, because they couldn't have sold it," he added.
"Once he had sold it, and was then an embarrassment in terms of not being able to cope with the job, they were determined to get rid of him."
In a television interview last September, Mr Robinson remarked that the DUP was "the most stable family party in Europe" and "a party that stands by its leadership".
In a second interview, he said the DUP was unlike other parties, adding: "It is a very special creation, it is a family much more than it is a political party.
"You don't have the back-stabbing and so forth that you have in other political parties."
However, those comments have been undermined by Mr Paisley's scathing attack on his party and former colleagues on TV.
He taunted Mr Robinson over the loss of his East Belfast parliamentary seat, claiming there was "a beast" within the DUP that was prepared to go forward to the destruction of the party.
Eileen Paisley made a series of personal jibes at Mr Robinson, branding his family a source of "sleaze" – an apparent reference to the sex scandal which engulfed his wife Iris four years ago.
Mr Paisley also made a remark – which some will see as a dig at his successor – about having a wife who still loves him.
Mr Allister said the Paisleys' astonishing criticism made a mockery of the DUP's claims of stability.
"I think that myth, and I stress the word myth, has well and truly exploded now," he added. "There was always a tension, let's be kind and call it that, between Ian Paisley and Peter Robinson.
"Paisley was pushed out – there is no doubt."