Sadiq Khan has warned Boris Johnson that the government’s plan to reduce immigration after Brexit will have “devastating consequences” for London.

The mayor of London demanded a major rethink of ministers' current plan for post-Brexit migration as he published new analysis showing that businesses in several key sectors would not be able to recruit enough staff.

Mr Khan has written to Priti Patel, the new home secretary, urging the government to reconsider its “flawed” immigration plans.

Speaking to the London Chamber of Commerce and Industry on Monday, he said that City Hall analysis had found that sectors including construction, social care and hospitality would all be hit hard by new restrictions on low-skilled immigration.

He called on Mr Johnson to lower the proposed salary threshold that, under the planned changes, prospective immigrants must meet to be considered a skilled worker.

Currently EU citizens are allowed to live and work in the UK under EU freedom of movement rules.

Under government plans for a new immigration system when freedom of movement ends post-Brexit, prospective immigrants would have to be earning at least £30,000 to qualify for a “skilled worker” visa. The condition currently only applies to non-EU migrants.

Using a salary threshold to determine skill levels is controversial because it disadvantages people with skills in low-paid sectors such as social care.

Mr Khan said the threshold should be lowered to £21,000 – the equivalent of the London Living Wage. The level at which the cut-off should be set is currently under review by the government's Migration Advisory Committee.

The London mayor said the capital should also be given the right to offer its own “fast-track" visas to people with skills needed in the city.

Mr Johnson has said he wants to restrict immigration even further by introducing an Australian-style points-based system where applicants are judged on the contribution they could make to the UK.

Almost one in seven jobs in London are currently filled by someone born in another EU country - a significantly higher proportion than in other UK regions.

The study by by the Greater London Authority (GLA) found that, under the government’s proposed immigration changes, employers would struggle to fill vacancies in occupations that account for around half of all the jobs in London, including nursery staff, cleaners, construction workers and other roles deemed as low-skilled.

Commenting on the findings, Mr Khan said: “London is known across the world for our openness, our diversity and our innovation and I’m proud to be the most pro-business mayor that this City has ever had. Making London a fairer city and ensuring that all Londoners get the opportunities that our city gave to me when I was growing up, is one of my core priorities as mayor.

“If the government’s proposed immigration changes go ahead, then I’m fearful for the impact they’ll have on the fabric of our city. The impact on the construction sector would make the housing crisis worse. And the impact on public services, including our schools and the NHS, could have devastating consequences for years to come.op

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He added: “The new prime minister should instead fully recognise the positive impact immigration and freedom of movement has had in London and the UK and immediately take steps to reform the immigration system in a way that enables us to unlock the potential of Londoners.

“If he is unable to do this, then he should let Londoners take back control and give City Hall the devolved powers that he previously called for as mayor.”

Mr Khan’s team said the proposed £30,000 threshold would prevent the recruitment of workers for roles that currently employ almost 150,000 EU citizens in London – a quarter of which are in construction and hospitality. Lowering the threshold to £21,000 would halve this figure, they claimed.

Under the mayor’s alternative immigration proposals, a database would be set up listing sectors in London where more workers were needed. Mr Khan is demanding new powers to fast-track visa applications from people with skills in these areas.

A Home Office spokesperson said: “As the prime minister has said, he wants our immigration system to help attract the brightest and best talent from around the world.