Rwanda said the United Nations refugee agency had lied when it told a British court this week that asylum seekers sent to the East African country could be moved on again to states where they risked torture or death.

Lawyers representing the United Nations High Commissioner for Refugees (UNHCR) said on Monday that Rwanda's asylum system was inadequate, as part of a challenge to the British government's policy to deport asylum seekers there.

The lawyers said the policy risked asylum seekers facing a banned process known as refoulement, building on past evidence that formed an important part of the British Supreme Court's reasoning when it ruled the British plan unlawful last year.

"UNHCR is lying," Rwanda's government spokesperson said in a statement on Tuesday.

"The organisation seems intent on presenting fabricated allegations to UK courts about Rwanda's treatment of asylum seekers, while still partnering with us to bring African migrants from Libya to safety in Rwanda."

The agency said it had always raised concerns about the risks refugees were exposed to from "externalisation", including refoulement.

"UNHCR ... finds that the UK-Rwanda Asylum partnership shifts responsibility for making asylum decisions and for protecting refugees," it said in a statement on Wednesday, declining further comment on grounds of related court action.

Rwanda's government said cases raised by the UNHCR lawyers in court had involved arrivals of those with legal status in other countries but who fell short of its entry requirements, or people leaving voluntarily.

Last week Britain said the first flight to Rwanda would take off on July 24, though that depends on Prime Minister Rishi Sunak's Conservatives winning general elections on July 4.

That looks unlikely, as the opposition Labour Party, leading by about 20 points in opinion polls, has pledged to scrap the plan if elected.