The colourful former speaker of Britain's House of Commons John Bercow said he has left the Conservatives to join the opposition Labour Party, launching a blistering attack on Prime Minister Boris Johnson. 

In an interview with the Observer newspaper published yesterday, the former MP said the Conservative Party under Johnson was "reactionary, populist, nationalistic and sometimes even xenophobic".

Bercow, who stepped down as speaker in October 2019 after 10 years, said he joined the Labour Party a few weeks ago because he shared its values.

"I am motivated by support for equality, social justice and internationalism. That is the Labour brand," he told the Observer.

Bercow described the prime minister as "a successful campaigner but a lousy governor."

"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. I think increasingly people are sick of lies, sick of empty slogans, sick of a failure to deliver," he told the newspaper.

Bercow served as a Conservative MP for Buckingham for 12 years before being elected speaker in 2009, becoming the youngest person to hold the role for 100 years.