Nigel Farage has pledged to abolish what the main route for immigrants to gain British citizenship, threatening tens of thousands of people legally living in the UK with deportation unless they meet strict rules.

The Reform UK leader said his plans would target the “Boriswave” – the wave of legal immigrants who arrived in the UK under post-Brexit rules established by Boris Johnson – which he described as the “greatest betrayal of democratic wishes certainly in anyone’s living memory”.

Unveiling his plans, Farage said 800,000 people who “tend to be young, tend to be low-skilled” were due to qualify for indefinite leave to remain in the UK over the next three to four years.

He said this group were “going to be a huge burden on the state”, that Britain was “not the world’s food bank” and that it was “not for us to provide welfare for people coming in from all over the world”.

Farage said he would entirely abolish indefinite leave to remain (ILR), which is open to people who have worked and lived in the UK legally for five continuous years, and their dependants. After a minimum of a year on that status, people can apply for British citizenship, though each stage comes with considerable fees.

Much of the increase in legal migration under Conservative governments starting with Johnson’s was because of schemes to settle refugees from Ukraine, Hong Kong and Afghanistan.

At a tetchy press conference alongside his policy chief, Zia Yusuf, on Monday morning, Farage was forced to qualify the announcement while refusing to answer questions on how much it would cost and who it would affect.

The proposals came under fire from Downing Street, which accused Farage of wanting to “foster division”, and from Sadiq Khan, the mayor of London.

Farage claimed the policy would save £234bn, a figure sourced from a Centre for Policy Studies report that has been withdrawn because of a dispute over the numbers. The thinktank said its figures “should not be used”.

Asked whether he would withdraw the £230bn figure in light of this, Farage said it was “without a doubt too low” and “underestimates things”.

Yusuf argued the true number of people who would be applying for ILR in the coming years was likely to be more than 800,000 because many of the people who have arrived after Brexit came from non-EU countries.

“Do you think French people and Swedish people and Finns are more or less likely to apply for ILR and ultimately citizenship in this country than people from India, Afghanistan or Pakistan? I think we all know the answer to that question,” Yusuf said.

Farage said that under a Reform government anyone who had indefinite leave to remain would have it rescinded and would need to reapply for a visa, throwing the lives and status of many families into uncertainty.

In response to questions from journalists, Yusuf clarified that the policy would not apply to EU citizens who have been granted settled status in the UK. But he said there were “a lot of EU nationals in this country who are drawing on universal credit, so you can expect Nigel’s government to open negotiations with the European Union specifically about the welfare aspect”.

Farage refused to specify whether the policy would apply to the Ukrainians and Hongkongers who moved to Britain under dedicated visa routes for refugees fleeing Russia’s invasion and China’s crackdown on civil liberties in Hong Kong.

Asked to substantiate his claim in Monday’s Daily Mail that most migrants rely on benefits, Farage said he was “firmly of the belief with research backing it up” that more than 50% of the people due to become eligible for ILR in the next few years “are not working, have not worked and in all probability will never, ever work”.

Instead of granting them ILR, Reform would make non-UK citizens continue to apply for visas with new high salary thresholds, though the party has not specified what those thresholds would be.

The Times reported they would be set at about £60,000, which would be a considerable increase on the current skilled worker visa, which requires people to earn £41,700 a year.

Those visas would not allow the people who have them to access NHS services or benefits and would require advanced English, with strict new limits on whether spouses or family members could join.

The changes would also mean that those applying for citizenship must wait at least seven years and prove a fluent level of English, as well as giving up citizenship of any other country.

Keir Starmer’s political spokesperson said the country was at a crossroads between “national renewal” and “the path of division and decline, which Reform wants to put the country on”.

She said: “Every week Nigel Farage sets out unrealistic, unworkable and unfunded plans. You’ve heard the prime minister talk about the politics of grievance that Reform thrives on. They don’t want to tackle the issues facing the country, they want to foster division.”

Labour has already announced plans to introduce tough restrictions on indefinite leave to remain, meaning most applicants must have been in the UK for 10 years before they can apply.

However, a Labour source disputed claims in the Telegraph that significant numbers of migrants with indefinite leave to remain were receiving benefits, saying it was only about 16% of all foreign claims. The majority – 60%, equating to about 770,000, according to government figures – are EU citizens who have settled status and whose access to benefits is guaranteed by the Brexit deal.

A Labour source said: “Farage’s not even half-baked announcement has already fallen apart. Yet again, Reform have no credible plan and their only answer is ‘don’t know’.”

Sadiq Khan, the mayor of London, said: “Thousands of Londoners have indefinite leave to remain. They have legal rights and are our friends, neighbours and colleagues, contributing hugely to our city. Threatening to deport people living and working here legally is unacceptable.”