Former Conservative MP and Commons speaker John Bercow has said his switching of allegiance to join the Labour Party is "not personal against Boris Johnson".
Mr Bercow told Sky News his decision to defect away from the Conservative Party is not to do with the PM, but added that Mr Johnson has "only a nodding acquaintance with the truth".
It came as Justice Secretary Robert Buckland said Mr Bercow "left the Conservative Party a long time ago".
Mr Buckland also accused the former speaker of having diminished his influence by coming out in support of Labour.
"I think him joining a political party actually has the effect of diminishing the force of his voice in politics, however strong he wants it to be," the justice secretary told Sky News.
Mr Bercow announced on Saturday that he had joined the Labour Party in recent weeks. He served as a Tory MP for Buckingham from 1997 until he was elected speaker in 2009.
Speaking to Sky News, Mr Bercow added that he "didn't have the slightest desire" to rejoin the Conservative Party after stepping down as speaker following a decade in the chair in 2019.
"If you're saying to me is it principally about Boris Johnson, the honest answer to that is no," he told Sky's Trevor Phillips on Sunday.
"It is not personal against Boris Johnson.
"I do think that he is someone who has only a nodding acquaintance with the truth in a leap year and I think that the utter contempt with which he has treated Parliament is lamentable and it has exacerbated the very strong feelings of resentment towards him - because I think a lot of people feel that is not the way to behave."
Mr Bercow also dismissed claims that his move to the Labour Party was motivated by wanting a peerage, something he was denied by Mr Johnson's government after standing down from his position as speaker.
"I've had absolutely no discussion whatsoever, either with Keir Starmer or any other member of the Labour leadership about that matter," he said.
"There has been no barter, no trade, no deal whatsoever."
Mr Bercow added: "It isn't in my mind, it's not part of the game plan, I haven't discussed it, I'm not waiting for it.
"What I'm motivated by is a commitment to equality, social justice and internationalism."
The former Commons speaker said he now has "a left of centre view" and believes Sir Keir would be a "vastly preferable" leader than Mr Johnson.
"Now I'm a private citizen, as Robert Buckland says, I'm entitled to take a political view. And my view is a left of centre view. I identify with Labour values, Labour principles, Labour policies," he said.
"The real issue in my mind is who has the vision of a more equitable society? Who thirsts to deliver social mobility?
"Who wants to better the lot of people less fortunate than a political leader?
"And on that calculus, Keir Starmer is vastly preferable to Boris Johnson."
Announcing his defection to the Labour Party on Saturday, Mr Bercow said he regards the Tories under Mr Johnson as "reactionary, populist, nationalistic and sometimes even xenophobic".
"I am motivated by support for equality, social justice and internationalism. That is the Labour brand," he told The Observer.
"The conclusion I have reached is that this government needs to be replaced.
"The reality is that the Labour Party is the only vehicle that can achieve that objective.
"There is no other credible option."
Mr Bercow's career was dogged by staff allegations of bullying, accusations he always denied.
He was also accused by Brexiteer MPs of being biased in favour of the Remain side of the Brexit debate.
The Labour leader's office declined to comment on Mr Bercow joining the party but shadow justice minister Karl Turner said he was unsurprised but "delighted" that his "friend" had joined Labour.
On the Conservative side, however, pensions minister Guy Opperman said: "Labour are welcome to Bercow."
A senior government source said: "This will surprise nobody and shows Labour is still the party of Remain."