A Briton who fought with an Islamist group in Syria and then faked his own death in the hope of being able to return home undetected has admitted to terrorism offenses in a London court.

Imran Khawaja, 27, of Southall, west London, traveled to a training camp in Syria in January last year and joined the Rayat al Tawheed group, which later became aligned with Islamic State, the Metropolitan Police said in a statement.

The group began posting violent propaganda online in an attempt to persuade others to come out and join them, it added.

"Khawaja was seen in a disturbing video posted on social media which includes a bag of severed heads," the statement said. "He appears in the footage with his face covered picking up one of the heads from the bag and showing it to the camera."

In May 2014, the group put out a message on social media claiming that he had been killed, alongside an image of two masked men holding an Islamic State flag.

Cmdr. Richard Walton, head of the SO15 Counter Terrorism Command, said: "Khawaja chose to become a terrorist, engaged in weapons training in a terrorist training camp and faked his own death in order to conceal his entry back into the U.K."

But he and his cousin, Tahir Bhatti, were arrested last June at the southeastern port of Dover.

Khawaja has admitted preparing to commit terrorist acts, receiving training in the use of firearms and attending a place for terrorist training.

Bhatti, 45, of Watford, north of London, pleaded guilty to helping him. Khawaja's plea only become reportable after Bhatti's admission on Tuesday at the Old Bailey court.

The men will be sentenced next month.