Britain paid Rwanda an additional £100-million in April, on top of £140-million it previously sent, as the bill for its contested plan to relocate asylum seekers to the East African country continues to rise.

The Rwanda scheme is at the centre of British Prime Minister Rishi Sunak's strategy to deter illegal migrants but as yet none have been moved there because of legal battles since the scheme was announced in 2022.

The divisive policy is now seen as a threat to Sunak's leadership - with an election expected next year - after his immigration minister resigned this week.

On top of the £240-million Britain has sent to Rwanda, London is also set to pay the East African country an additional £50-million next year, according to a letter published by the British interior ministry on Thursday.

The revelations about the growing cost of a policy - which legal experts have warned could yet fail - was slammed by the opposition Labour party and will likely to draw fresh criticism from some lawmakers within Sunak's own party.

"Britain can’t afford more of this costly Tory chaos & farce," Labour's shadow interior minister Yvette Cooper said on social media platform X.

But the new minister for legal migration, Tom Pursglove, justified what he called the £240-million "investment" on Friday, saying that once the Rwanda policy was up and running it would save on the cost of housing asylum-seekers in the UK.

"When you consider that we are unacceptably spending £8-million a day in the asylum system at the moment, it is a key part of our strategy to bring those costs down," Pursglove told Sky News.

The money sent to Rwanda would help its economic development and get the asylum partnership with the UK up and running, Pursglove added.

The payments to Rwanda were not linked to a treaty the two countries signed on Tuesday, the interior ministry letter said.

The treaty seeks to respond to a ruling by Britain's Supreme Court that the deportation scheme would violate international human rights laws enshrined in domestic legislation.

"The Government of Rwanda did not ask for any payment in order for a Treaty to be signed, nor was any offered," the letter said.

Sunak appealed to his Conservative lawmakers on Thursday to unite behind his Rwanda plan after Robert Jenrick quit as immigration minister on Wednesday, saying the government's draft emergency legislation to get the scheme up and running did not go far enough.