A former British student who has returned from Syria after fighting along the Islamic State has told German interrogators that the dreaded terror group is planning major simultaneous attacks across the UK and Europe.

Harry Sarfo, a 27-year-old ISIS recruit who spent time in the terrorist group's stronghold of Raqqa earlier this year, told German security officials that virtually all foreign fighters are being asked if they will return to their home countries to carry out attacks.

"They want something that happens everywhere at the same time," Sarfo told the security officials, according to an account published in German news magazine Der Spiegel.

Sarfo, a German national who converted to Islam in London and is linked to British terrorists in Syria, claimed he was repeatedly approached to volunteer for such a mission. He, however, told recruiters that he was not willing to carry out a domestic atrocity and later fled ISIS.

The magazine last week named him only as 'Harry S' in accordance with German media convention relating to suspects in live investigations. However, The Sunday Times has established Sarfo's full identity from British sources.

Any information he has provided to the German authorities will almost certainly be shared with British intelligence agency MI5 and Scotland Yard's counter-terrorism unit.

Although he grew up in Germany to Christian parents of Ghanaian origin, Sarfo is said to have moved as a teenager to London with his mother after becoming involved in petty criminality in Bremen city.

He appears on the electoral roll at addresses in Enfield, north London, between 2007 and 2012. But it is thought he may have been in the capital from as early as 2005 and enrolled at a technical college.

Following Sarfo's conversion to Islam, he adopted the name Bilal and appears to have become involved with hardline fundamentalist groups in London. Last year, he travelled on pilgrimage to Mecca with at least one other British man.

He returned to Germany before heading out to join ISIS in Syria this year. During his time there, Sarfo says he 'frequently' heard people talking about attacks on the West.

Sarfo was fleetingly seen at the start of a film carrying an ISIS banner. His lawyer Udo Wurtz told the magazine that Sarfo had not taken part in any executions.

"He is a lackey who allowed himself to be misled by the propaganda of ISIS. He wants to come clean," said Wurtz.

Estimated 800 British extremists travelled to Syria out of which 450 are believed to have come back to the UK.

In October, MI5 chief Andrew Parker warned that ISIS is planning mass terror attacks in Britain.