The UK has given Rwanda a further £100m this year as part of its deal to relocate asylum seekers there.
The payment was made in April, the Home Office's top civil servant said in a letter to MPs, after £140m had already been sent to the African nation.
Sir Matthew Rycroft said another payment of £50m was expected next year.
The revelation came hours after Rishi Sunak vowed to "finish the job" of reviving the plan after the resignation of his immigration minister this week.
The scheme to send some asylum seekers to Rwanda for processing, in order to deter people from crossing the English Channel in small boats, was first announced by then-Prime Minister Boris Johnson in April 2022.
But it has been repeatedly delayed by legal challenges and no asylum seekers have been sent from the UK so far.
Until now it was known that the government had spent at least £140m on the policy. Sir Matthew had previously refused to disclose updated figures, saying ministers had decided to set out the costs annually.
But in a letter published on Thursday to Dame Diana Johnson, chairwoman of the Home Affairs Committee, and Dame Meg Hillier, chairwoman of the Public Accounts Committee, he disclosed the full cost of the policy so far.
On Friday morning, Mrs Hillier told BBC Radio 4's Today programme that the figures were only revealed after repeated inquiries, and that there have been no updates when the situation changes.
She said it is "unconscionable" that MPs are expected to vote on the emergency Rwanda legislation next week "without understanding fully what the costs are so far, what they're expected to deliver, and what the costs are going forward."
"It almost looks like the government's got something to hide," she said.
Sir Matthew stressed that the extra payments were not linked to the new treaty signed this week between UK and Rwanda as part of the government's attempt to amend the policy, which was ruled unlawful by the Supreme Court last month.
A Home Office spokesperson would not go into specifics on what the money would be spent on but said it was going towards the economic development and growth of Rwanda.
The payment was made when Suella Braverman was home secretary, though allies of hers say it was signed off by the prime minister.
Labour branded the revelation of the extra costs "incredible", with shadow home secretary Yvette Cooper adding: "How many more blank cheques will Rishi Sunak write before the Tories come clean about this scheme being a total farce?" she said.
'Merry-go-round of legal challenges'
Earlier on Thursday, Mr Sunak held a press conference where he urged Tory MPs to back his plan.
The prime minister was speaking a day after immigration minister Robert Jenrick resigned over the government's revised policy, saying he believed it was destined for failure.
Mr Sunak insisted the new emergency legislation set out by the government would end the "merry-go-round of legal challenges" over the flights of some asylum seekers to Rwanda.
The bill compels judges to treat Rwanda as a safe country and gives ministers the powers to disregard sections of the Human Rights Act. But it does not go as far as allowing them to dismiss the European Convention on Human Rights, as some on the right of the Conservative Party have called for.
The bill faces opposition from MPs in different factions of the Conservative Party when it returns to Parliament next week.
Also on Thursday, former Home Secretary Suella Braverman reiterated that it would fail to "stop the boats" and called on the government to fully exclude international law.
The task of steering the bill through Parliament falls to Michael Tomlinson, who was appointed illegal migration minister on Thursday.
He will work alongside Tom Pursglove, the minister for legal migration, after the prime minister split Mr Jenrick's vacant role in two.
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2023-12-08 07:40:27Z
CBMiL2h0dHBzOi8vd3d3LmJiYy5jby51ay9uZXdzL3VrLXBvbGl0aWNzLTY3NjU2MjIw0gEzaHR0cHM6Ly93d3cuYmJjLmNvLnVrL25ld3MvdWstcG9saXRpY3MtNjc2NTYyMjAuYW1w
