Ministers today dismissed 'diminished' John Bercow after he announced he is joining Labour - saying Boris Johnson only has a 'nodding acquaintance with the truth'.
Justice Secretary Robert Buckland shrugged off the barbs as the ex-Speaker stepped up his attack on the PM in a television interview.
Mr Bercow - who was a Tory before becoming Commons chair in 2009 - caused fury on the Conservative benches for what they saw as bias in the handling of Brexit wrangling.
He finally quit the supposedly impartial post in November 2019, but has now announced that he is a Labour member.
Appearing on Sky News this morning, Mr Bercow said that Mr Johnson only had a 'nodding acquaintance with the truth on a leap year'.
He has also branded the 'reactionary, populist, nationalistic and sometimes even xenophobic' - but denied that he is angling for a peerage from the Labour Party.
Mr Buckland said during his own interview that Mr Bercow was 'a private citizen' and 'entitled to make decisions about his politics'.
'But 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,' he added.
Mr Buckland said he disagreed 'totally' with the former Tory MP's characterisation of the party today being 'xenophobic'.
A senior Government source said: 'This will surprise nobody and shows Labour is still the party of Remain.'
Justice Secretary Robert Buckland (left) shrugged off the barbs as ex-Speaker John Bercow (right) stepped up his attack on the PM in a television interview
Westminster sources confirmed that John Bercow, accused as Speaker of repeated pro-Remain prejudice, has signed up to the party within the last four weeks, with his name recently appearing on a 'new joiners' list
The move to Labour completes an extraordinary political journey for Mr Bercow, originally elected as a Tory MP, from self-confessed 'hard-Right' politics in his youth to Labour member.
It also marks a sharp break with the tradition that Commons Speakers, who give up party political affiliations on taking that post, stay impartial by retiring to the Lords as crossbench peers.
Mr Bercow told the Trevor Phillips programme on Sky News: 'It's 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 I think it has exacerbated the very strong feelings of resentment towards him, because I think a lot of people feel that's not the way to behave.
'Telling the truth in and to Parliament matters, circumventing Parliament is wrong, treating Parliament with disdain is objectionable, but no, I have over a long period evolved my political thinking.
'I wasn't a member of a party throughout my tenure as speaker, because it would have been quite wrong to be.
'I didn't take a view. I sought to facilitate the House of Commons to express its view and all individual members to put their points across because that's the responsibility of the speaker.
'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.'
Mr Bercow dodged saying whether he would accept a peerage if Labour put him forward for one again.
The 58-year-old was last year denied elevation to the Lords amid claims - which he rejected – that he had bullied staff during his time overseeing Commons proceedings.
Tory critics pointed out last night that it was ex-Labour leader Jeremy Corbyn who proposed him for the Lords, and suggested that joining the party now was a bid to revive his peerage hopes.
But Mr Bercow said: 'I've had absolutely no discussion whatsoever, either with Keir Starmer or any other member of the Labour leadership about that matter.
'There has been no barter, no trade, no deal whatsoever.
'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.'
Mr Bercow, who infuriated Brexiteer MPs as Speaker by revealing he had voted Remain, declined repeated requests to comment to the Mail on Sunday last week.
But in an interview with the Left-leaning Observer newspaper, the ex-Speaker admitted he has joined Sir Keir Starmer's party, and delivered a savage attack on Boris Johnson's Tories, dismissing the party he belonged to for many years as 'reactionary, populist, nationalistic and sometimes even xenophobic'.
Mr Bercow said he joined Labour because he now shares its values and sees it as the only means to removing the current Government.
He also branded Mr Johnson 'a successful campaigner but a lousy governor'.
Labour sources insisted there was 'no deal' for Mr Bercow to be nominated for a peerage by Sir Keir, or for any other party post.
His move to Labour completes an extraordinary political journey for Mr Bercow, originally elected as a Tory MP, from self-confessed 'hard-Right' politics in his youth to Labour member.
Mr Bercow was elected Speaker in 2009 amid claims even then that he was ready to ditch many of his Tory principles to win the votes of Labour MPs, anxious to install a Speaker ahead of the 2010 general election who would cause trouble if the Tories won power.
He was understood to have won the support of many Labour MPs despite once having been a member of controversial hard-Right Tory pressure group the Monday Club.
In his autobiography, Mr Bercow goes out of his way to disown his 'Monday Club dalliance', saying that it was a 'most shameful decision' and referring to how he had been 'zealously pursuing my ugly brand of hard-Right politics'.
Mr Bercow added that he had quit the group when he was 21 and had 'for decades now subscribed to a much more mainstream and progressive view of multiracial Britain'.
He was elected Buckingham MP in 1997 and in his Commons' maiden speech lauded Margaret Thatcher – who died in 2013 – as 'the world's greatest living statesman'.
Mr Bercow, 58, was last year denied a peerage amid reports that he was under investigation over allegations – which he rejected – that he bullied staff during his time overseeing Commons proceedings
Last night, Tory MP and former Brexit Minister David Jones asked how that would go down with the ex-Speaker's new Labour colleagues.
Mr Jones added: 'John Bercow is simply removing all doubt about the pro-Labour, anti-Brexit leanings he has very obviously harboured from some time. In fact, given how biased he was as Speaker, perhaps the only surprise is he didn't join the socialists earlier.
'I can only presume he is hoping that by paying his membership fee, Sir Keir will follow through on Jeremy Corbyn's offer to send him to the Lords.'
Tory Brexiteer Peter Bone said: 'I think John Bercow will be very much at home in the Left-wing, elitist, anti-Brexit organisation that is the Labour Party under Sir Keir.'
But pro-Remain Labour MP Neil Coyle hailed the move last night, saying: 'I personally would welcome any other former Tories who want to join.'