Bercow retired last week after a decade as speaker that saw him become a central player in Britain's Brexit drama.
Mr Bercow, who was frequently accused of siding with Remain MPs in Parliament, insisted he was "always fair in the chair" but said: "Brexit is the biggest foreign policy mistake in history".
Mr Bercow - who was a Conservative MP before being elected to the Speaker role in 2009 - was often sharply criticised by members of his former party for his decisions regarding Brexit debates in the Commons.
"I think that Brexit is the biggest foreign policy mistake in the post-war period".
Mr Bercow, who left his role as Commons Speaker on October 31 after 10 years in the chair, said the United Kingdom leaving the European Union "doesn't help us" and the United Kingdom is better-off within the "power bloc".
He continued: "The honest answer is that I don't think it helps the UK".
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Speaking to Anadolu Agency following an event with foreign journalists in London, John Bercow said: "I made my position clear on the issue in February 2017 and nothing has changed since". I respect his right to his view, which is a view very widely shared, but there is an alternative view equally that is very widely shared, and my own personal opinion is that Brexit does not help us.
On Monday, Mr Bercow was formally replaced as Commons Speaker by Sir Lindsay Hoyle.
Hoyle, 62, was elected to Parliament in 1997, has served as one of the three deputy speakers since 2010 and is widely popular and respected by colleagues.
Sir Lindsay said he wants to ensure that backbenchers are still able to hold ministers to account while enabling parliamentary business to be conducted in "the best possible way".
Sir Lindsay's election was greeted with a flurry of tributes from MPs on his first full day as Speaker.
Mr. Bercow announced that he intends to step down in September.