Britain’s government will announce on Tuesday (7 March) plans for a new law barring those entering the country through unofficial routes from claiming asylum, in a bid to stop tens of thousands of migrants arriving on its shores in small boats.

Prime Minister Rishi Sunak has made stopping the boat arrivals one of his five key priorities after the number of migrants arriving on the south coast of England soared to more than 45,000 last year, with around 90% of them applying for asylum.

The new legislation will mean that anyone who arrives on small boats will be prevented from claiming asylum and deported to so-called safe third countries, according to government officials, who asked not to be named.

“We have an unacceptable situation, depriving people who genuinely need our help and that is simply not fair, so we have to take away the incentive to jump the queue by coming here illegally and stop the boats,” the prime minister’s spokesman said on Monday.

The Refugee Council charity said tens of thousands of genuine refugees who would previously have been granted asylum would be “locked up like criminals” under the plans, which would “shatter” Britain’s commitments under the UN refugee convention.

Anger over immigration in some areas has played a defining role in British politics over the last decade, and deployed successfully by campaigners as a tool to fuel support for Brexit ahead of the 2016 referendum.

Controlling immigration was the third-most important issue for voters after the economy and the running of the health service, polling by YouGov in November found. The poll found 87% of the public thought the government was handling the issue badly.

Opposition parties and charities have questioned whether the latest plans would be any more effective than previous attempts to deter people from making the Channel crossing, which has proved profitable for human traffickers and perilous for migrants. Four drowned in December when their boat capsized.

Last year, former Prime Minister Boris Johnson agreed a deal to send tens of thousands of migrants – many having made the journey from Afghanistan, Syria or other countries torn apart by conflict – more than 4,000 miles away (6,400 km) to Rwanda.

But the first planned deportation flight was blocked in June by a last-minute injunction granted by the European Court of Human Rights (ECHR), and the strategy’s lawfulness was subsequently challenged at London’s High Court.

The court subsequently ruled it lawful in December, but opponents are seeking to appeal that verdict. It is expected the legal battle will end up in the UK Supreme Court and may not be resolved for months.

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Sunak will meet with the French President Emmanuel Macron on Friday when he is expected to ask for more co-operation in tackling the gangs involved in cross-Channel people smuggling.

Once the small-boat crossings have ended, Britain would be prepared to create more legal routes for asylum seekers, Sunak said last year. But he has not said what these will be.