John Bercow says Boris Johnson will be no better than a bank robber if he refuses to obey the law to prevent a crash-out Brexit, in a fresh attack on his conduct.

Delivering a lecture, the outgoing Commons speaker lashed out at the prime minister’s threat to ignore parliament’s instruction to seek a further Article 50 extension – warning it would undermine the entire rule of law.

“It is astonishing that anyone has even entertained the notion. It would be the most terrible example to set to the rest of society,” Mr Bercow said.

“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.”

Mr Bercow also warned Mr Johnson he would fail to wriggle out of the legislation to prevent a no-deal Brexit on 31 October, vowing “neither the limitations of the existing rulebook or ticking of the clock” would defeat MPs.

If the government comes close to disobeying the law, Bercow said he “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.

And he proposed a new parliamentary powers act to rein in rogue prime ministers in future – while arguing the Brexit chaos had made the case for the UK to have a written constitution.

Protesters take to streets after Boris Johnson suspends parliament Show all 11 1 /11 Protesters take to streets after Boris Johnson suspends parliament Protesters take to streets after Boris Johnson suspends parliament Anti Brexit protesters march on Whitehall EPA Protesters take to streets after Boris Johnson suspends parliament Many of the protesters carried placards AP Protesters take to streets after Boris Johnson suspends parliament A demonstrator wears a Boris Johnson mask during a London rally AFP/Getty Images Protesters take to streets after Boris Johnson suspends parliament An anti-Brexit demonstrator wields an EU flag during a London rally. AFP/Getty Images Protesters take to streets after Boris Johnson suspends parliament The protesters also marched to Trafalgar Square, where they blocked traffic AP Protesters take to streets after Boris Johnson suspends parliament John McDonnell addressed crowds in Whitehall. PA Protesters take to streets after Boris Johnson suspends parliament A small group of Brexit supporters attempted to stage a counter protest in central London. AP Protesters take to streets after Boris Johnson suspends parliament Three people were arrested in London, including a woman who sat on the road near Trafalgar Square. AP Protesters take to streets after Boris Johnson suspends parliament In Oxford, locals and students gathered outside Balliol College, Boris Johnson's alma mater, to protest. PA Protesters take to streets after Boris Johnson suspends parliament Thousands of people gather in central Glasgow to protest against the prime minister. AFP/Getty Images Protesters take to streets after Boris Johnson suspends parliament Jeremy Corbyn spoke to protesters in central Glasgow. Getty

The attack follows Downing Street’s threat to trigger a Supreme Court battle, rather than comply with the law to secure a further Brexit delay if necessary.

It will be a race against time for MPs to force Mr Johnson to Brussels before Halloween, or to bring him down in a vote of no-confidence and send a replacement.

Delivering a speech for the Bingham Centre for the Rule of Law, Mr Bercow warned the government would lose all “moral force” to enforce the law if it was seen to break the law itself.

“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?

“What conceivable moral force do the public’s elected representatives have in seeking to tackle antisocial behaviour, in prosecuting with greater vigour and imagination and relentlessness the fight against knife crime, in arguing that the state must protect itself against all sorts of nefarious illegality, if we are to treat for a moment with the proposition that it might be in order, in the name of some higher cause, to disregard a law enacted by parliament?”

Mr Bercow has become a champion of the Commons efforts to block a no-deal Brexit, before announcing, on Monday, that he will quit as Speaker on 31 October.