The cost of Rishi Sunak’s troubled Rwanda asylum scheme could soar to half a billion pounds, plus hundreds of thousands more for each deportee, an investigation by the public spending watchdog has found.

The Home Office has so far refused to say how much more money, on top of the £290 million already confirmed, the UK has agreed to pay Kigali under the stalled plan, but a National Audit Office report has revealed millions more in spending including £11,000 for each migrant’s plane ticket.

Critics said the “staggering figures” reveal “the extortionate bill the taxpayer will have to pay the Rwandan government for an unworkable and inhumane scheme” and “the national scandal the Tories have been trying to hide”.

The Government has already paid Kigali £220 million under the Economic Transformation and Integration Fund designed to support Rwanda’s growth, even as flights have remained grounded amid a series of legal setbacks.

It was also already known that an extra £50 million was earmarked for the partnership for next year. But the NAO revealed the same sum will also be sent to Rwanda in 2025 and in 2026, taking the cost to £370 million.

On top of that, once the first 300 migrants have been relocated to Rwanda, ministers have agreed to put another £120 million into the fund, lifting the total to £490 million.

In addition, an extra £20,000 will be paid to Rwanda for every asylum seeker relocated there, according to the NAO report.

The Home Office will also separately hand Kigali nearly £151,000 per person to cover asylum processing and integration costs, such as accommodation, food, healthcare and education, over five years if they stay in the country.

If they decide to leave, the UK would halt payments for that person, but would still give Rwanda a one-off £10,000 to help facilitate their departure.

The Government has already sent Kigali an advance payment of £20 million to cover these processing and operational costs for the first arrivals.

No asylum seeker who has arrived in the UK via unauthorised means has so far been removed to Rwanda under the Conservatives’ flagship asylum policy, although the Prime Minister has said he wants flights to start “as soon as practically possible”.

The Home Office has also already incurred direct costs of £20 million in establishing the agreement, including set-up charges for escorting migrants on the one-way trip and training facilities, legal fees and staff.

This is expected to rise to £28 million by the end of the year, according to the watchdog.

The department estimates that future costs will include £11,000 per asylum seeker for flights to Rwanda, covering the charter of non-commercial planes and fuel, the NAO said.

Some £12.6 million will also be spent on training people to escort migrants in the first year and £1 million per year thereafter, and £1 million annually in staff costs.

(The NAO's) shocking analysis shows the costs of the failed Rwanda farce are even higher than previously thought

The NAO investigated the Rwanda plan’s costs between January and February after the chairs of the Public Accounts and Home Affairs committees raised concerns about the lack of information available to Parliament.

The initial five-year deal runs to April 2027, with payments potentially continuing until 2033, according to the NAO, which did not consider its value for money.

The Rwanda policy is a key plank of Mr Sunak’s plan to “stop the boats”, as he believes it will be a deterrent to further Channel crossings.

No migrants have so far been removed because of the legal challenges that resulted in the Supreme Court finding the scheme unlawful.

Mr Sunak is trying to revive the policy by passing legislation deeming Rwanda a safe country and ratifying a new treaty with Kigali. The Rwanda Bill is currently making its way through a House of Lords that is hostile to the scheme.

Labour’s shadow home secretary Yvette Cooper said: “This report reveals the national scandal the Tories have been trying to hide. Its shocking analysis shows the costs of the failed Rwanda farce are even higher than previously thought.

“In order to send less than 1% of UK asylum seekers to Rwanda on a few symbolic flights, the taxpayer will be forced to fork out over half a billion pounds – with no ability to recover any of the money already sent. This is the equivalent of nearly £2 million per person sent.

“Rishi Sunak has staked his position on this scheme. He must account for this fiasco.”

Home Affairs Committee chairwoman Dame Diana Johnson said: “These are staggering figures. For all its rhetoric about ensuring value for money in the asylum and immigration system it is unclear how schemes such as Rwanda or Bibby Stockholm achieve that. Huge initial outlay and ongoing costs raise serious questions about how this can be cost effective, even compared to high hotel accommodation costs.

“What we are left with is a very expensive programme the Government hopes may offer a deterrent to those seeking to cross the Channel in small boats. Yet, there is little evidence for this either…

“This also does little to allay the serious concerns … about the lack of openness on the cost implications of the Rwanda scheme from the Home Office. For a scheme whose importance is apparently self-evident, we would expect the evidence base to be far clearer, not presented in dribs and drabs and getting worse every time.”

Refugee Council chief executive Enver Solomon said: “These figures reveal the extortionate bill the taxpayer will have to pay the Rwandan government for an unworkable and inhumane scheme that will not deter people seeking protection on our shores.”

A Home Office spokesperson said: “It is vital we respond to illegal migration with bold, long-term solutions. Our Partnership with Rwanda offers just that.

“Doing nothing is not without significant costs. Unless we act, the cost of housing asylum seekers is set to reach £11 billion per year by 2026. Illegal migration costs lives and perpetuates human trafficking, and it is therefore right that we fund solutions to break this unsustainable cycle.

“We have a strong relationship with Rwanda and both sides remain absolutely determined to deliver on this Partnership. Once the Safety of Rwanda Bill and Treaty are in place, we will focus on getting flights off the ground.”