'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.'
Read more: Brexit: John Bercow pledges 'creativity' to stop no-deal
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