The government spent a further £100m in the 2023-24 financial year while flights remained grounded amid a series of legal setbacks - on top of the £140m previously paid out.

According to a letter from the Home Office to committee chairs, ministers expect additional costs of £50m in the coming year, which would bring the total to £290m.

Politics - latest: Rwanda vote not about leadership, says Rishi Sunak

It came just hours after Mr Sunak vowed to "finish the job" of reviving his plan to deport some asylum seekers to Kigali - despite the prospect of a bitter parliamentary battle.

Home Office official Matthew Rycroft wrote to Home Affairs Committee chair Dame Diana Johnson, and Public Accounts Committee chair Dame Meg Hillier, on Thursday.

His letter said: "Ministers have agreed that I can disclose now the payments so far in the 2023-24 financial year.

"There has been one payment of £100m, paid in April this year as part of the Economic Transformation and Integration Fund mentioned above.

"The UK government has not paid any more to the government of Rwanda thus far.

"This was entirely separate to the treaty - the government of Rwanda did not ask for any payment in order for a treaty to be signed, nor was any offered."

Labour described the revelation as "incredible" - with shadow home secretary Yvette Cooper saying: "How many more blank cheques will Rishi Sunak write before the Tories come clean about this scheme being a total farce?

"Britain simply can't afford more of this costly chaos from the Conservatives."

But defending the growing bill, migration minister Tom Pursglove told Sky News: "When you consider that we are unacceptably spending £8m a day in the asylum system at the moment, it is a key part of our strategy to bring those costs down so I think this is the right investment to make that will help us achieve those objectives of saving lives at sea, stopping people drowning in the Channel, as well as getting those costs under control in a way I think taxpayers across the country want to see."

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Rishi Sunak facing political fight of his life

He also signalled the government could be open to compromises with rebel Tory MPs to push through emergency legislation, which declares that Rwanda is a safe destination for asylum seekers.

The Bill has divided backbenchers with Conservative hardliners arguing it does not go far enough and pressing for it to effectively override international law, while MPs on the moderate side of the party are said to be "very nervous" about the implications of the proposed law.

Despite the public splits, Mr Pursglove said: "I think there is a unity of purpose among Tory MPs that action does need to be taken that we do need to deliver on this.

"There will be parliamentary debates, there will be opportunities for people to bring amendments, the house will consider them in the normal way and as ministers we will engage constructively with parliamentarians around any concerns that they have and handle that in the way that we would any other piece of legislation.

"We will engage with colleagues around concerns that they have, but I am pretty clear that this plan is the right plan and we are determined to see it through.

"This is the right approach to move this issue forward."

He added: "I do think parliamentarians across the House should come together to back this.

"If you really want to stop the small boat crossings, this is such a critical part of the plan I think all MPs should be getting in behind it."

Mr Sunak has insisted his new law would end the "merry-go-round of legal challenges".

MPs will get their first chance to debate and vote on the Safety of Rwanda (Asylum and Immigration) Bill on Tuesday.

The prime minister dismissed suggestions he will make it a confidence vote, meaning that MPs would have the whip withdrawn if they defied him.

Under the government's plan first unveiled in April 2022, people who arrive in the UK by irregular means - such as on small boats - could be sent on a one-way trip to Rwanda, where the Kigali government would decide on their refugee status.