A Briton who fought in Syria with an Islamist group then faked his own death and tried to return home undetected admitted terrorism offences at the Old Bailey.

Imran Khawaja, 27, of Southall, west London, travelled to a training camp in Syria in January 2014 and joined a group called Rayat al-Tawheed, which became aligned with Islamic State, the Metropolitan police said in a statement.

The group posted 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 Met 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 claimed on social media that he had been killed, alongside an image of two masked men holding a flag associated with Islamic State.

Commander 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 UK.”

But he and his cousin Tahir Bhatti were arrested last June at Dover.

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

Bhatti, 45, of Watford, admitted 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.

At a previous hearing, prosecutor Mark Dawson said that by 18 March last year, Bhatti, who was referred to in a coded message as “butterbean”, was well aware that Khawaja was in Syria and that he was clearly training with a view to fighting in Syria.

The court was shown three photographs from Khawaja’s phone that allegedly reflected what Bhatti knew about his activities.

The first showed Khawaja in battlefield clothes and a balaclava sitting on a tank with a rifle. Another pictured him with a child sporting a distinctive tassled hat – the same hat and clothing worn in a number of postings by a terrorist group. The last photo showed the defendant at a training camp with an assault rifle, the court heard.

By June last year, Bhatti had convinced his cousin to return to the UK saying his parents were not well and he should return home.

Defending, Joel Bennathan QC had argued: “He was doing what Muslim families are meant to do – tell them to desist and come out of Syria.”

But Dawson said the “road trip” was carefully orchestrated with the use of codewords in messages. There was talk of being “in a club” and needing “clothes” because of the “puke” and that a “doorman” was not letting him out. “Mr Bhatti does not bat an eyelid at this terminology. It is known and understood,” said Dawson.

The “club” meant a camp. The “doormen” were those running it. “Puke” related to battlefield material – he needed new clothes.

The court had heard that another feature of the case was the use of a communications app called Telegram – a secure and encrypted system that could not be intercepted.

At an Old Bailey hearing last month, Khawaja admitted preparation of terrorist acts on or before 26 January last year as well as attending a terrorism training camp between January and 4 June last year.

He also admitted receiving weapons training and possessing a firearm for terrorist use. But Khawaja denied a charge of soliciting murder between 25 January and 4 June last year with and against people unknown.

His pleas can only now be reported since his co-defendants admitted terror charges and the prosecution decided not to pursue a trial.

On 23 December last year, Asim Ali, 33, from Ealing, west London, pleaded guilty to making £300 available to Khawaja while knowing or having reason to suspect it might be used for the purposes of terrorism. But he denied another charge of preparing a terrorist act between January and June 2014.