The diplomat added embassies are also frustrated by the lack of coordination between departments on the issue.

“The FCDO always says talk to the Home Office,” the person said. “The Home Office says we should speak to FCDO — and the Cabinet Office says it has nothing to do with them.”

A third diplomat called it “a tricky situation,” saying European capitals are worried prisoners fresh out of “British jail can just pop to an airport in their home country, and are allowed to walk freely.”

They said capitals would want more “information about the person being transferred,” but that the “Brits treat it as if there is nothing to confirm with us, and that it’s just a unilateral deportation.”

But prison reform campaigners point out that many of these offenders would face deportation at the end of their sentences anyway. The changes bring forward the point at which foreign prisoners become eligible for removal, as opposed to the outcome.

“It is up to the receiving country to decide if the prisoner should serve any prison time,” said Mark Day, deputy director at the Prison Reform Trust. “Some foreign nationals who are deported may not be required to serve any form of sentence. For others, the sentence served may be more punitive than the one they would have experienced in the U.K.”