In a scathing letter to the home secretary – seen by The Independent – the London mayor said the proposals would be a “fundamental affront to British values of tolerance and respect”.

Mr Khan’s comments come as Downing Street said free movement for EU citizens “as it currently stands” will end on the 31 October Brexit deadline – despite a replacement system not yet up and running.

No 10 said the structure allowing European Union citizens to freely live and work in the UK would “look different”, with changes including tougher checks to prevent foreign criminals entering the country.

In his letter to Priti Patel on Monday, Mr Khan said he was “very concerned” about plans to end free movement on day one of a no-deal Brexit. “I urge you strongly to reconsider,” he said.

The London mayor continued: “I fear that such a reckless change to our immigration system would have far deeper consequences too.

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“Three million EU citizens living in the UK right now – one million of whom are Londoners – make an enormous contribution to our economic, cultural and civic life.

“By putting the rights and freedoms of EU citizens in the UK at risk in such a careless and brutal fashion, these plans would be a fundamental affront to British values of tolerance and respect.

“These plans risk providing encouragement and cover for perpetrators of hate crime and discrimination in the UK who may be heartened by the government discriminating against people on the basis of their nationality.”

Mr Khan also claimed that to proceed with the plans – despite warnings of “another Windrush” scandal – could “only be described as cruel”.

The 3 Million group, which campaigns for the rights of EU citizens in the UK, said in a statement: “The idea of ending freedom of movement abruptly on October 31 in case of no deal is reckless politics.

“It hollows out the prime minister’s unequivocal guarantee to EU citizens he has given only three weeks ago. Ending freedom of movement without putting legal provisions in place for those EU citizens who have not yet successfully applied through the settlement scheme will mean that millions of lawful citizens will have their legal status removed overnight.

“We have been calling for the settlement scheme to be a declaratory registration scheme, so all EU citizens who have made the UK their home are automatically granted status, as promised by those in government.”

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A Home Office spokesperson said: “EU citizens and their families still have until at least December 2020 to apply to the EU settlement scheme, and 1 million people have already been granted status.