Britain's government lawyers told the country's top court on Monday there was a serious and pressing need for its plan to deport migrants to Rwanda, one of the Prime Minister Rishi Sunak's key policies, as they sought to overturn a ruling which declared it unlawful.

In June, London's Court of Appeal concluded that the scheme to send tens of thousands of migrants more than 6,400 kilometers to East Africa was not lawful, saying Rwanda could not be treated as a safe third country.

The court ruled that those sent to Rwanda would be at risk of being sent home where they could face persecution despite having a legitimate asylum claim. This would make the policy unlawful under Britain's Human Rights Act (HRA), which made the European Convention on Human Rights (ECHR) part of British law.

The decision dealt a massive blow to Sunak's pledge to stop thousands of migrants from arriving in small boats on the English south coast.

At the start of three days of hearings, the government's lawyer James Eadie told the UK Supreme Court the appeal concerned "a policy to which the government attaches considerable importance".

"There is a serious and pressing need to take effective steps that will act as a deterrent to those undertaking the perilous and sometimes life-threatening journey, typically across the Channel from a safe country, normally France," Eadie said.

"There is, we submit, a strong public interest in that legitimate, indeed key, policy aim," he added.

Lawyers representing eight of the asylum seekers involved in the case said in court documents there were "critical defects" in Rwanda's system for determining the status of refugees, also referencing the Rwandan government's "capture of the judiciary" and the repression of political dissent.

Legal teams representing migrants from Syria, Iraq, Iran, Vietnam and Sudan also want the judges to conclude the scheme itself is unlawful on a wider basis.

The political stakes are high and the outcome, with a final ruling likely by year-end, could have far-reaching consequences.

Sunak, whose Conservatives are trailing by about 20 points in opinion polls ahead of an election expected next year, has made a vow to "stop the boats" one of his five priorities as he seeks to turn around his party's fortunes.

Surveys show that immigration, a key factor in the 2016 referendum vote for Britain to leave the European Union, remains a major concern for voters, and that a majority of the public think the government is handling the issue badly.

The government's first planned Rwanda deportation flight had been due to leave in June last year, but was blocked at the last minute by an injunction from the European Court of Human Rights until all UK legal action had been concluded.

Many in Sunak's party want Britain to pull out of the ECHR to prevent any repeat, and in a speech to party faithful last week Home Secretary (interior minister) Suella Braverman labelled the Human Rights Act, "the Criminal Rights Act".

"Know this; I will do whatever is necessary to stop the boats," Sunak told the Conservative conference.

(With input from Reuters)