The interior ministry official said he had no evidence that the plan to send asylum seekers to Rwanda would have a deterrent effect.

In a letter published over the weekend, Matthew Rycroft, the department’s permanent secretary, said that as evidence was not available to justify the plan he could not be sure he would get his money’s worth for the taxpayer.

The release of the letter coincided with Justin Welby, the Archbishop of Canterbury, using his Easter sermon to say the principle behind the plan “cannot bear God’s judgment”.

Home Secretary Priti Patel has justified Rwanda’s proposal on the grounds that the number of people crossing the Channel in small boats in the hope of seeking asylum in the UK has risen sharply and the prospect to be sent to Rwanda would have a deterrent effect. , disrupting human traffic.

She also claimed that the costs of the scheme would be a “drop in the ocean” compared to the long-term costs of the continued increase in Channel crossings by small boats.

But in his letter, Rycroft said he couldn’t be sure that argument was correct.

He told Patel: “The value for money of the policy depends on its effectiveness as a deterrent. The evidence for a deterrent effect is highly uncertain and cannot be quantified with sufficient certainty to provide me with the necessary level of assurance as to value for money.

Rycroft wrote to tell Patel she should issue a ministerial instruction for the policy to move forward. This is a rare procedure used when civil servants cannot justify a policy on sound grounds for public spending and decide to flag their concerns so that ministers must take a formal policy decision to override them.

In his letter, Rycroft stressed that he wasn’t saying the policy wouldn’t be a deterrent — just that it was impossible to know anyway.

“I don’t think enough evidence can be obtained to demonstrate that the policy will have a strong enough deterrent effect for the policy to get value for money,” he said. “That doesn’t mean that [the policy] cannot have the appropriate deterrent effect; just that there is not enough evidence for me to conclude that it will.

Over the weekend, the Home Office also released the text of Patel’s response to Rycroft, in which she confirmed she was issuing a Ministerial Instruction.

She said the asylum system was already costing the government £1.5billion a year and, although she admitted it was not possible to accurately model the deterrent effect of Rwandan policy , “we are convinced that this policy is our best chance of producing this effect”. .

She also said it would be “unwise…to allow the absence of quantifiable and dynamic modeling…to delay the implementation of a policy that we believe will reduce illegal migration, save lives and ultimately break the model.” business of smuggling gangs”.

Britain has promised to pay Rwanda an initial sum of £120 million to launch the programme. It will also pay a sum for each person resettled in the country, although these figures were not disclosed.

The Home Office is launching a £100,000 social media advertising campaign to inform potential asylum seekers, in their own language, of the policy that could send them to Rwanda if they cross the Channel in a small boat is already in place.

In his Sunday sermon, Welby said there were “serious ethical questions” about the proposal.

“Details are for politics and politicians. The principle must withstand the judgment of God and it cannot,” he said.

“He cannot bear the weight of our national responsibility as a country shaped by Christian values, because outsourcing our responsibilities, even to a country that seeks to do well like Rwanda, is against nature. of God, who himself took responsibility for our failures.

In response, Tory MP Andrew Bridgen accused Welby of “a bit of naivety”, telling Sky News he did not believe the Archbishop’s views were “in line with the views of the country”.