Jeremy Corbyn and Sadiq Khan have called on the government to scrap plans to immediately end freedom of movement from the European Union in the event of a no-deal Brexit.
The Labour leader said the policy could be a scandal like “Windrush on steroids” and urged the Home Office not to oppose restrictions on EU nationals in UK.
In a letter to Home Secretary Priti Patel, Mr Khan said ending free movement without any transition before a new immigration system comes into force would be “careless and brutal”.
The calls come after reports that officials have warned ministers against an immediate change in the event the UK leaves without an agreement in place at the end of October.
Mr Khan, the London mayor, told Sky News: “I think it’s wrong. I think it’s callous and it’s cruel to use people as bargaining chips.”
Mr Corbyn said: “It is an utterly ludicrous position she [the home secretary] has adopted.
“Does that mean that a European Union national living in this country, possibly as a doctor, a nurse, a trauma surgeon, all kinds of things, goes home to see their family in Germany or Czech Republic or wherever else, they are not allowed back into this country? Is this another Windrush, on steroids? It is a ludicrous proposal.
“We rely on a lot of European nationals to maintain our industries, maintain our education services.”
He added: “I’d just say to Priti Patel, cut the nonsense, and don’t impose this again.”
Number 10 has said the system allowing EU citizens to freely live and work in the UK will “look different” after 31 October.
Changes will include tougher checks to prevent foreign criminals entering the country, Downing Street said.
But the Times has reported that ministers were warned that radically altering the system then could leave the UK facing “another Windrush” – a reference to the immigration scandal that saw Amber Rudd quit as home secretary.
This saw British residents, who came to Britain from the Caribbean in the decades after the Second World War, experience issues as a result of tightened immigration requirements.
Named after the cruise ship that brought one of the first large groups of West Indians to Britain, anyone who entered the UK before 1973 was legally entitled to live in the country.
But it emerged that long-term UK residents were denied access to services, held in detention or even deported despite living legally in the country for decades.
According to the Times, Home Office officials produced a discussion paper presented at a ministerial no-deal Brexit meeting last week which warned that the freedom of movement plan could present a “handling and reputational risk” for the government.
A source told the newspaper that concerns were raised about an “interim” immigration system, with fears that this would be impossible to enforce because the government and employers would be unable to distinguish between new arrivals and those already in the country.
Instead, the report recommended that free movement should continue until the new immigration system was ready in January 2021 to give “maximum certainty” to EU citizens and employers.
But a Downing Street spokeswoman said on Monday: “Freedom of movement as it currently stands will end on 31 October when the UK leaves the EU.
“So for example we will introduce, immediately, much tougher criminality rules for people entering the UK.
“Details of other changes immediately on 31 October for a new immigration system are currently being developed.
“The prime minister has obviously been clear that we want to introduce an Australian-style points-based immigration system.”
Further details will be set out by Mr Johnson and Ms Patel shortly, she added.
Conservative MP Alberto Costa, who has been campaigning on the issue of EU citizens’ rights, said the government risked “legal chaos” if it did not legislate to protect their rights.
“If we end, suddenly and abruptly, free movement of people without replacing it with a new immigration system that would bring legal chaos and undoubtedly a tsunami of litigation against the UK government,” he told Sky News.
Downing Street has insisted that EU citizens currently resident in the UK will not be prevented from re-entering the country after trips abroad, although it is not clear how checks will be carried out.
The system which allows EU citizens to apply for settled status will not be changed and the two million who have not yet completed the process will not be prevented from entering the UK by the ending of free movement, Number 10 has said.
“The Home Office have set out that no one eligible for settled status will be unable to re-enter the UK when free movement ends and they have obviously been doing a significant amount of work to communicate how you apply,” a government spokeswoman said.
The 3 Million group, which campaigns for the rights of EU citizens in the UK, said the ending of free movement was “reckless politics”.
“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,” the group said in a statement.