The policy, which was denounced by rights groups and even King Charles after it was announced in April, would involve Britain sending tens of thousands of migrants who arrive on its shores more than 4,000 miles away (6,400 km) to Rwanda.

Announcing the court's decision, judges Clive Lewis and Jonathan Swift said it was lawful for Britain to make arrangements with the Rwanda government to send asylum seekers to the country for their asylum claims to be determined there.

The ruling is a relief for Sunak who is fighting growing industrial action, high levels of inflation, and is under increasing pressure from his own members of parliament and the public to deal with the small boats.

In one of his first major policy announcements, Sunak set out a strategy to clamp down on illegal immigration and said he wanted to restart the flights to Rwanda even though the policy has been criticised by lawmakers in all the main political parties and by the United Nations.

King Charles reportedly described the plan in private as "appalling".

On Monday, Amnesty International said it was "bitterly disappointed" by the ruling.

"We remain gravely concerned that the government’s Rwanda deal seriously undermines international refugee law and rides roughshod over the rights of people seeking asylum in the UK," said Steve Valdez-Symonds, the group's Refugee and Migrant Rights Director.

Over the last decade migration has often dominated Britain's political discourse and is likely to feature heavily in the campaign for the next national election, expected to take place in 2024.

Figures show more than 40,000 - a record number - have come from France this year, many having made the journey from Afghanistan, Iran or other countries suffering war to travel across Europe and on to Britain to seek asylum.

PARTIAL VICTORY

The judges said that the plan complied with the government’s obligations under the 1998 Human Rights Act and the 1951 United Nations refugees convention.

"The court has concluded that, it is lawful for the government to make arrangements for relocating asylum seekers to Rwanda for their asylum claims to be determined in Rwanda rather than in the UK," the judges said.

After the ruling Britain's interior minister, Suella Braverman, said the government's focus was now on proceeding with the deportation policy "as soon as possible" and it was ready to defend against any further legal challenges.

Yolande Makolo, a spokeswoman for the Rwandan government, welcomed the court's decision, and said her country wanted to offer migrants the chance to build a new life.

The first planned deportation flight to Rwanda 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 government victory will not mean flights can take off straight away because there may be further appeals in the British courts and the ECHR injunction imposed during the summer prevents any immediate deportations until the conclusion of legal action in the United Kingdom.

Detention Action and Asylum Aid - who challenged the policy alongside asylum seekers from Syria, Iraq, Iran, Albania, and Vietnam - said they would consider lodging an appeal against the court's decision.

In a partial win for the claimants, the judges said the government must consider properly the circumstances of each individual claimant and referred them back to be reconsidered.

The judges said a further hearing will take place on Jan. 16 to determine any applications for permission to appeal against the court's decision.

Paul O’Connor of the Public and Commercial Services Union, which represents government officials and was involved in the legal challenge, said he was pleased individual asylum seekers could challenge their removal.

"Anyone who thinks this litigation is going away any time soon should probably think again."