The Brexit process should be halted to allow time for a general election or another referendum if MPs reject Theresa May's deal, Sadiq Khan is urging.
The London Mayor is using a visit to the Republic to argue against the prospect of a "devastating" no-deal Brexit.
Mr Khan, a prominent pro-EU campaigner during the 2016 referendum, suggests the UK should withdraw the Article 50 notification which began the countdown to the March 29, 2019 Brexit date to allow more time to "resolve this mess".
In a speech in Dublin he said: "The worst possible outcome - which remains a grave concern given the political uncertainty in Westminster - is that the UK leaves the EU with no Brexit deal at all.
"This would not only be devastating for London's economy, but would actually hurt businesses and economies across Europe, including here in Dublin, and increase the chance of a hard border on the island of Ireland.
"In my view, the prospect, the very notion, of a no-deal Brexit must be taken off the table by the British government.
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"It's just too dangerous to leave as a possibility.
"That's why, today, I'm calling on Theresa May to withdraw Article 50 if the British Parliament rejects her deal next week, which is looking extremely likely.
"This move would be the single best way to guarantee that we avoid falling off the cliff edge.
"It would stop the clock that is ticking down towards a no-deal Brexit, and it would provide the breathing space to decide how we resolve this mess.
"If the British Prime Minister refuses to take this precautionary step - we would continue heading towards a no-deal Brexit in just a few months' time - and would risk us having to explain to future generations why the Government of the day knowingly put our economy, our prosperity and our place on the world stage in such grave peril."
