John Bercow has insisted MPs will have their say over whether the UK leaves the EU without a deal on 31 October.
The Speaker dismissed claims from some Brexiteers that because a no-deal exit is the default position in law, it will “inevitably” happen if no agreement is reached by then with Brussels.
Instead, he said there was “much debate to be had” and it was “unimaginable” that Parliament would be sidelined.
The October deadline was set after MPs repeatedly rejected Theresa May’s plan.
The Commons has already voted to block a no-deal exit, but that vote was non-binding.
Several of them, including Boris Johnson and Dominic Raab, have said they would be willing to leave the EU on a no-deal basis. Others, though, like Jeremy Hunt and Rory Stewart, say that would be unacceptable.
The winner of the contest – not yet formally under way – is expected to be confirmed in July and will also become the UK’s next prime minister.
Speaking at the Brookings Institution in Washington, US, Mr Bercow said: “There is a difference between a legal default position and what the interplay of different political forces in Parliament will facilitate.
“The idea that Parliament is going to be evacuated for the centre stage of debate on Brexit is simply unimaginable. The idea the House won’t have its say is for the birds.”
He continued: “The idea that there is an inevitability of a no-deal Brexit would be a quite wrong suggestion. There is no inevitability whatsoever about that.”
Mr Bercow said there was still “a lot still to be said… and policy to be determined.”
But he would not predict the outcome, and said Article 50 – the mechanism to leave the EU – was triggered under the last Parliament and “no Parliament can bind the hands of its successor”.
“I believe passionately that Parliament must do what Parliament thinks is right,” said Mr Bercow.
“MPs have a duty to do what they think is right in terms of voice and vote.”