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 and potentially resettlement, 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.
The figures were disclosed in a letter to Dame Diana Johnson, who chairs the home affairs select committee and and her fellow Labour Dame Meg Hiller, who chairs spending watchdog the public accounts committee.
Dame Meg told BBC Radio 4's Today programme the full cost of the policy so far had only been revealed after repeated inquiries.
"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.
Legal migration minister Tom Pursglove said the money was being spent to ensure the Rwanda policy was "robust".
He added that the scheme was "key" to reducing the amount the government spends on housing migrants in UK hotels - currently £8m a day.
Robert Jenrick's immigration minister role was split into two following his resignation. Michael Tomlinson is the illegal migration 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?"
"It's basically £100m for every home secretary trip to Rwanda," she said.
The Home Office has said Rwanda has an initial capacity to take 200 people a year, but there are plans to increase that number when the scheme begins.
The department has also estimated the cost of sending someone to a safe country - not specifically Rwanda - is £169,000, compared to £106,000 if they remain in the UK.
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 Times has reported that senior government lawyers had warned No 10 the emergency legislation could still allow migrants to lodge challenges against their removal to Rwanda.
Speaking to BBC Radio 4's Today programme, Mr Pursglove insisted the legislation would "close off the routes to try to frustrate the appeals process".
On Sky News he said ministers would "engage constructively with parliamentarians around any concerns that they have".