The number of illegal migrants arriving in the UK must fall by 10,000 for the Rwanda scheme to be value for money, the Home Office admits

The number of migrants arriving illegally in Britain must fall by about 10,000 for the Rwanda policy to prove value for money, the head of the Home Office has admitted.

Sir Matthew Rycroft told MPs yesterday that a one-third reduction in small boat crossings from last year's figure of 29,437 would be needed given the cost of the deportation scheme, which will reach £290million by next year.

He revealed that the government in Kigali will be handed another £50m of the total almost as soon as the new law aimed at getting flights off the ground is passed by Parliament this week.

Sir Matthew was asked by members of the Public Accounts Committee what would happen to the 40,000 people already in Britain left 'in limbo' when the latest legislation aimed at stopping the boats takes effect as they will not be eligible to claim asylum and will be in a long queue to be sent to Africa.

He insisted Rwanda is ready and able to take as many migrants as Britain can send, however.

'It's the responsibility of the third of our Rwanda partners to provide that accommodation and they have assured us, and we have tested that over the over the month indeed over the last two years, that they have that capability to accommodate the people who will be relocated.'

Sir Matthew and his team were also blasted by the MPs over the soaring costs of two former Ministry of Defence sites being turned into accommodation for illegal migrants.

He accepted their allegation that the Home Office had got the sums 'so so wrong' for the set-up and refurbishment of Scampton and Wethersfield, both originally estimated at £5m each and now put at £27m and £49m respectively.

'We made an assessment about how much the set up costs would be, which in the case of both of those sites was woefully short. So there was a very significant optimism bias.'