Mr Bercow, who has long been accused of being pro-Remain by Brexiteers, said he thought Brexit was “the biggest mistake of this country after the war”.
Speaking at a meeting with the Foreign Press Association in London, Mr Bercow, 56, said: “I'm no longer the Speaker, I don't have to be impartial, so if you ask me if I think Brexit is good for our international standing, the honest answer is not, I don't think so.”
He added: “I don’t think it helps the UK. Brexit is the biggest mistake of this country after the war. I respect Boris Johnson but Brexit doesn’t help us. It’s better to be part of the EU power bloc.”
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.”
With a General Election taking place in December, Tory Mr Bercow refused to reveal which party he will vote for and predicted that Brexit “won’t be resolved any time soon”.
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Mr Bercow’s successor as Speaker, Sir Lindsay Hoyle, was elected to the position this week, and began his time in the chair by promising to end the "bear pit" atmosphere of confrontation in the House of Commons.
The 62-year-old former deputy speaker was elected after securing more than half of votes in the fourth round of voting by MPs on Monday.
Speaking to BBC Radio 4's Today programme, he said his priority would be to encourage tolerance and respect for all in the House.
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".
He said: "I think my style will be about trying to get business through the House in the best possible way but making sure that backbenchers are heard and they are going to hold the executive to account," he said.
"That's going to be my new role."
Praise for new Speaker
Sir Lindsay’s election was greeted with a flurry of tributes from MPs on his first full day as Speaker.
Former minister Crispin Blunt, who had accused Mr Bercow of “bias” over Brexit, was the first Tory backbencher to be called during Foreign Office questions, and joked: “I hope I can now get called, which is agreeable.”
Foreign Secretary Dominic Raab told the Speaker: “You will not have an easy task but I’m confident that with your technical expertise, your long experience and good humour, you’ll do an absolutely superb job.”
Sir Lindsay defeated former Labour colleague Chris Bryant in the final round by 325 votes to 213.
Mr Bercow left the role after a decade which has been viewed as a time of reform but also controversy.