London Mayor Sadiq Khan has warned that a hard Brexit could rip Britain apart
Mr Khan said that privileged access to the European single market is "critical" for the UK's capital and urged business leaders to make the case to ministers that a hard Brexit would be bad for their companies.
Describing a hard Brexit – in which the UK leaves both the single market and customs union – as a "lose-lose situation" for Britain and Europe, Remainer Mr Khan warned that today's politicians will have to answer to future generations if they put their prosperity and their place in the world "in such peril".
Speaking to an audience of business leaders at the World Economic Forum in Davos, Switzerland, Mr Khan was due to say: "Securing privileged access to the single market must be the top priority for the negotiations. It's critical for London. Nothing else will do. It can't be brushed aside – as it was yesterday."
Responding to Mrs May's speech, in which she said she would not seek to retain single market membership and may quit the customs union, the Mayor warned: "A hard-line approach to Brexit may hold the Conservative Party together, but it could rip Britain apart.
Mr Khan said that privileged access to the European single market is "critical" for the UK's capital
"And if we continue on this path – towards a hard Brexit – we risk having to explain to future generations why we knowingly put their economy, their prosperity and their place on the world stage, in such peril."
A hard Brexit would cut Europe off from its only truly global financial centre
Calling on businesses and organisations across Europe to make the case to their political leaders for a workable Brexit deal, Mr Khan was expected to say: "A hard Brexit would cut Europe off from its only truly global financial centre. This would be bad news for Europe as well as Britain. So a hard Brexit really would be a lose-lose situation.
Mr Khan is confident that London will remain Europe's leading financial centre
"Tell your political leaders that a hard Brexit deal is not in the best interests of your company. This won't be easy. But I'm confident that despite the Prime Minister's rhetoric, there is still a sensible deal to be done."
Mr Khan will say that he is "confident" that London will remain "Europe's leading business hub" after EU withdrawal and that the city will continue to be "the cultural, social and economic capital of Europe".
But he will warn that the EU risks further withdrawals unless it addressed the widespread "unease" in several countries over the changes to ordinary people's lives in recent decades.
Sadiq Khan elected London Mayor
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Sadiq Khan sworn into office
Theresa May speaking to the World Economic Forum in Davos
Economic and social divides were "not unique to Britain" and will spread further in the absence of a "concerted response" from European leaders, he said.
And he warned: "If many of your countries held an EU referendum tomorrow, it could go the same way as ours. This is an existential threat to the EU – that we must combat together.
"One of the most crucial tasks for us now is to take pro-active steps to build stronger and more integrated communities. And to ensure that everyone benefits from our globalised economy. The alternative is division, protectionism and isolationism."