The UK’s allies in Syria have released European Islamic State suspects back to the terror group through secret prisoner swap deals, the Telegraph can reveal, raising concerns that jihadists are returning to the battlefield and could re-enter Europe.

The fighters returned so far included French and German nationals, but there are three British prisoners held by the Syrian Democratic Forces (SDF) who could potentially be involved in future deals.

The West has refused to take responsibility for dozens of European fighters being held by the SDF, which says it does not have the capacity to keep Isil prisoners indefinitely.

The group has struck three agreements with Isil for the exchange of militants and their families with captured SDF fighters, according to people with knowledge of the negotiations and relatives of detained Isil members.

The first was made in February and involved some 200 fighters; most were Chechen and Arab but the group also contained a number of French and at least one German.