John Bercow has vowed "creativity" in Parliament if Boris Johnson ignores the law designed to stop a no-deal Brexit.
The Commons Speaker also said in a speech that the only possible Brexit was one backed by MPs.
A new law, passed before the suspension of Parliament, forces the PM to seek a delay until 31 January 2020, unless a deal or no-deal exit is approved by MPs by 19 October.
The PM has said he would rather be "dead in a ditch" than ask for a delay.
Delivering a lecture in London, Mr Bercow said: "Not obeying the law must surely be a non-starter. Period."
He said it would be a "terrible example to set to the rest of society".
"The only form of Brexit which we will have, whenever that might be, will be a Brexit that the House of Commons has explicitly endorsed," he said.
"Surely, in 2019, in modern Britain, in a parliamentary democracy, we - parliamentarians, legislators - cannot in all conscience be conducting a debate as to whether adherence to the law is or isn't required."
He called it "astonishing" that "anyone has even entertained the notion".
If the government comes close to disobeying the Act, the MP said that Parliament "would want to cut off such a possibility and do so forcefully".
"If that demands additional procedural creativity in order to come to pass, it is a racing certainty that this will happen, and that neither the limitations of the existing rule book nor the ticking of the clock will stop it doing so," he added.
He also compared any attempt to shun the law over the extension of Article 50 - the process by which the UK leaves the EU - with a bank robber excusing their crime by giving their stolen cash to charity.
"One should no more refuse to request an extension of Article 50 because of what one might regard as the noble end of departing from the EU as soon as possible, than one could possibly excuse robbing a bank on the basis that the cash stolen would be donated to a charitable cause immediately afterwards," he said.
The law forcing the PM to seek a delay unless MPs vote for a deal or no deal received royal assent on Monday, the final day that MPs sat in this session.
Parliament was prorogued for five weeks in the early hours of Tuesday.
The UK is due to leave the EU on 31 October, but the new law passed by MPs could extend that deadline.
On Thursday, Mr Johnson insisted the UK "will be ready" to leave the EU by the current deadline without an agreement "if we have to".
In response to the publication of the government's Yellowhammer document, an assessment of a reasonable worst-case scenario in the event of a no-deal Brexit, Mr Johnson reiterated it was "the worst-case scenario".
"In reality we will certainly be ready for a no-deal Brexit if we have to do it and I stress again that's not where we intend to end up," Mr Johnson said.
Mr Bercow has announced he will stand down as Commons Speaker and MP at the next election or on 31 October, whichever comes first.
'No reason to be optimistic'
The Speaker's warning came as the EU's chief Brexit negotiator told political leaders in the European Parliament on Thursday that he could not say whether contacts with the UK government would result in a deal by mid-October.
Michel Barnier, in a speech to MEPs, suggested that negotiating a new withdrawal agreement remained uncertain despite discussions between Mr Johnson's team and the EU.
"I cannot tell you objectively whether contacts with the government of Mr Johnson will be able to reach an agreement by mid-October," he said.
"While we have previously reached an agreement, as far as we can speak, we have no reason to be optimistic."
He added: "We will see in the coming weeks if the British are able to make concrete proposals in writing that are legally operational."
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