Former Commons Speaker John Bercow has called Brexit "the biggest foreign policy mistake in the post-war period".
Giving his opinion to the Foreign Press Association in London, he told journalists he no longer had to "remain impartial" after stepping down from the chair after 10 years.
Mr Bercow was accused by some Brexit-backing MPs of siding with Remainers during his time as Speaker.
But he told the event he believed he was "always fair" to MPs on all sides.
Mr Bercow announced his intention to stand down in September, with his exit due to coincide with 31 October Brexit deadline - now delayed until 31 January.
His deputy, Sir Lindsay Hoyle, was elected to take over his post on Monday.
With his new-found freedom to express his opinion, journalists questioned him about his time as Speaker.
And asked whether Brexit would be good for the UK's global standing, Mr Bercow told the event: "The honest answer is no."
He added: "I think that Brexit is the biggest foreign policy mistake in the post-war period, and that is my honest view."
Mr Bercow also said, "with total certainty", Parliament would be debating Brexit for at least the next five years - if not the next 15 - and that was "blindingly obvious".
During his decade in the role, Mr Bercow gave unprecedented powers to backbenchers to hold ministers to account and made controversial and far-reaching procedural decisions at key stages of the Brexit process.
Answering his critics, Mr Bercow told the event he had "always treated the Brexiteers in a fair way" and "always treated the Remainers in a fair way".
He added: "I will assert to anybody that will listen until my dying day that I have been impartial in the chair, pro-Parliament and impartial in the chair."
But Brexit Party MEP Rupert Lowe said it was "disgraceful" someone with Mr Bercow's views was "allowed to referee our Parliament for so long", adding: "The whole establishment is geared against Brexit."