Mr Bercow said he regards the Tories under prime minister Boris Johnson as “reactionary, populist, nationalistic and sometimes even xenophobic”.
Having stepped down as speaker after 10 years in 2019, he announced on Saturday that he joined Labour in recent weeks.
“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 served as a Tory MP for 12 years until he was elected speaker in 2009.
In September 2019, he announced his intention to stand down from the role, as well as MP for Buckingham, and vacated the speaker’s chair at the end of October that year.
Labour nominated him for a peerage after the Tories declined to do so, breaking with the long-standing convention that speakers are elevated once they retire.
Asked if there is the possibility of being recommended for a peerage by Keir Starmer, Mr Bercow told the Observer: “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. – PA