Mr Bercow, an ex-Tory MP, said he had joined Labour a few weeks ago because he is motivated by the party’s values.
Lashing out at the prime minister, he said the Conservatives today are a “reactionary, populist, nationalistic and sometimes even xenophobic” party.
Professing his support for Keir Starmer’s party, Mr Bercow said: “I am motivated by support for equality, social justice and internationalism. That is the Labour brand.”
After more than a decade in the chair, Mr Bercow stepped down from the speaker role in October 2019, following a tumultuous period in which he oversaw the final stages of the Brexit agreement.
He was the subject of intense anger from Brexit-supporting MPs who believed a series of rulings he made during his time as speaker favoured remain-supporting MPs.
The PM later broke with hundreds of years of precedent by refusing to give him a peerage when he stepped down and left parliament.
He had also been accused of bullying by Commons clerks, which he denied. He claimed last year there was a “conspiracy” to stop him getting a seat in the House of Lords.
In an explosive interview with The Observer, the former MP for Buckingham said the current government needed to be replaced and that Labour “is the only vehicle that can achieve that objective”.
He also accused the PM of having no interest in anybody but himself, despite his so-called “levelling-up” agenda.
“He is a successful campaigner but a lousy governor,” he told the paper. “I don’t think he has any vision of a more equitable society, any thirst for social mobility or any passion to better the lot of people less fortunate than he is.”
He added: “I think increasingly people are sick of lies, sick of empty slogans, sick of a failure to deliver.”
Asked if there is the possibility of being recommended for a peerage by Sir Keir Starmer, Mr Bercow said: “There has been no such discussion and I have asked for no such thing. This isn’t about revenge. That is not what motivates me.”
The Labour leader's office declined to comment on Mr Bercow joining the party.
Shadow justice minister Karl Turner said he was unsurprised his “friend” had joined Labour, but was “delighted that he has”.
On the Conservative side, however, pensions minister Guy Opperman said it was “from bad to worse for” Sir Kier, adding: “Labour are welcome to Bercow.”
A senior government source said: “This will surprise nobody and shows Labour is still the party of remain.”
Additional reporting by Press Association