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Sadiq Khan today told business chiefs that key members of Theresa May’s Government were “leading us ever closer to the cliff edge” by pushing for a hard Brexit.

In a major speech, the Mayor warned that leaving the single market would inflict huge economic damage on London and the rest of the UK, and send out the wrong signal to investors.

He urged ministers to bring in a work permit scheme — the so-called London visa — so the capital can continue to attract talent from around the world.

Boris Johnson, David Davis and Liam Fox — known as the Three Brexiteers — have been the most vocal proponents of hard Brexit and are thought to be the ministers Mr Khan has in his sights.

The Mayor wants Britain to retain access to the single market to safeguard economic prosperity and help maintain London’s position as a world-leading financial centre. He warned the alternative would mean relying on standard World Trade Organisation rules, which could mean costly barriers to trade.

He was expected to tell the Confederation of British Industry: “There’s been a lot of tough talk from the Government in recent days with what looks like a lurch towards a ‘hard Brexit’ approach. It seems some people at the heart of Government are willing to lead us ever closer to the cliff edge ... The unnecessary rush to trigger Article 50 [starting off the Brexit process] — is sending all the wrong signals to investors during this uncertain period.”

Mr Khan, who was a leading figure in the Remain camp, warned it would be “disastrous” for the British economy if the UK left without reaching a deal on access to the single market.

High-profile Brexiters have argued that leaving the single market would “liberate” Britain to champion free trade while retaining control over mig-ration. However, a leaked draft Government report today warned Treasury coffers would take a £66 billion annual hit and 9.5 per cent would be cut from GDP if Britain goes for a hard Brexit.

Mr Khan said: “We need the Government to really be on the side of business as we go into negotiations — something that seems lacking at the moment.”

City Hall is already working on the idea of a London “visa” after being urged by business leaders to help stop an exodus of skilled foreign workers.

A government spokesman said: “We want the best outcome for Britain. That means pursuing a bespoke arrangement which gives British companies the maximum freedom to trade with and operate in the single market, and enables us to decide for ourselves how we control immigration.”