London's Court of Appeal will rule on Thursday if a British plan to deport asylum seekers to Rwanda is lawful, a decision that could make or break Prime Minister Rishi Sunak's pledge to stop migrants from arriving by boat.

Under a deal struck last year, Britain's government plans to send tens of thousands of asylum seekers who arrive on its shores more than 4,000 miles (6,400 km) to the East African country.

The first planned deportation flight was blocked a year ago in a last-minute ruling by the European Court of Human Rights (ECHR), which imposed an injunction preventing any deportations until the conclusion of legal action in Britain.

In December, the High Court ruled the policy was lawful, but that decision is being challenged by asylum seekers from several countries along with human rights organisations.

The Court of Appeal is due to deliver its judgment at about 0900 GMT.

A victory for the government will not mean immediate flights because there may be a further court appeal and the ECHR injunction prevents any deportations until three weeks after the conclusion of British legal action.

If the judges rule the plan is lawful, a government official said the flights could begin later this year if the courts reject any applications for further legal challenges.

However, if the asylum seekers lose on Thursday, but win the right to appeal to the Supreme Court, the first flights are unlikely to take off this year.

Sunak sees the deportation plan as central to deterring asylum seekers arriving from Europe.

He has made "stop the boats" one of five priorities, and is hoping a fall in arrivals might help his Conservative Party pull off an unexpected win at the next national election.

Sending each asylum seeker to Rwanda will cost on average 169,000 pounds ($213,450), the government said this week.

The main opposition Labour Party called the assessment a "complete joke" and the Scottish National Party accused the government of spending an "astronomical" sum on deportations while failing to help Britons with the rising costs of mortgages and food bills.

Opponents of the legislation say the government’s policies are about driving political support and will not solve underlying issues.

There say they are currently no legal routes for most asylum seekers fleeing war or persecution to apply for refugee status to enter Britain, so many see the dangerous small boat crossings as their only option.

Britain's upper house of parliament on Wednesday inflicted a series of defeats on the government's legislation designed to speed up the process of deporting people to Rwanda.

The House of Lords made four changes to the legislation, including increasing protections for victims of modern slavery.

Last year, a record 45,755 people came to Britain in small boats across the Channel, mainly from France. Over 11,000 have arrived so far this year, a similar rate to the first half of 2022.