A former grammar school pupil has pleaded guilty to travelling to Syria to join Islamic State, despite previously claiming he had spent his time there playing computer games.

Shabazz Suleman had just completed his AS-levels at a grammar school in High Wycombe, and had a place to study international relations at university when he vanished while on a family holiday to Turkey in 2014, aged 19.

He later claimed he had volunteered with Humanitarian Relief Foundation (IHH), a Turkish non-governmental organisation, before he was arrested by Turkish security forces.

Suleman was then traded to IS as part of a controversial prisoner swap to secure the release of 47 Turkish diplomats captured by IS from their embassy in Mosul, northern Iraq, in November 2014.

In an interview with Sky News in 2017, Suleman insisted he spent most of his three years in terrorist territory playing PlayStation and riding his bike.

He spoke about how he’d gone into hiding to try to avoid fighting, sitting in various houses in Raqqa playing GTA (Grand Theft Auto) or Metal Gear Solid on a PlayStation and having “a normal life in IS territory”.

In Syria, he assumed the name Abu Shamil al-Britani and celebrated the Charlie Hebdo killings in messages in early 2015, warning: “There’s so many brothers just waiting for the order to do attacks on the West.”

Among the images of Raqqa that he posted to Twitter at that time was one of an alleged spy, beheaded and crucified, although he insisted that he did not take part in torture or killings.

However, by June 2015 he had become disillusioned, saying, “I never thought I was being brainwashed until I saw the way they treat other Sunnis” – referring to fellow Muslims.

He was arrested and spent time in an IS prison under the stadium in Raqqa city centre for a month, but after watching people being tortured and beaten up, he “gave in” and agreed to stay with IS.

Suleman said he was transferred to the military police, keeping order among the civilian population but insisted he was mostly office-bound, playing games on his laptop or on guard duty at checkpoints.

He claimed he then went into hiding “watching Netflix, House of Cards mostly” and that he had bought a Playstation console to play video games.

Suleman later said he was prepared to serve a prison sentence in order to return home, and criticised the murders of 22 concertgoers in the Manchester Arena bombing.

He was captured by Liwa al-Shimal, a militia aligned with the Free Syrian Army, in Jarabulus on the Syrian border with Turkey, in October 2017.

Speaking after leaving IS, Suleman told Sky News: “I take responsibility. I was with ISIS, I was with a terrorist organisation. But I didn’t kill anyone, I hope I didn’t oppress anyone.

“I did have Kalashnikov and a military uniform, but I didn’t hit anyone, I didn’t oppress anyone, if you understand. I was there with military police but like I said, I was in the office.”

He was arrested after returning to Heathrow in October 2021 and charged with a string of terror offences.

He was due to face trial at the Old Bailey in central London next month, but on Friday he pleaded guilty to preparing acts of terrorism by travelling from the UK to Turkey in order to join IS in Syria in August 2014.

The defendant, now 27, was also charged with being a member of IS, a proscribed organisation, between 2014 and 2017, and receiving training in the use of firearms.

Prosecutor Duncan Atkinson KC said his guilty plea addressed the two other charges which he asked to be left on the court file.

Referring to the defendant’s basis of plea, Judge Mark Lucraft KC agreed and noted Suleman had gone to join with IS “even if not becoming a member of it”.

Suleman was remanded in custody and sentencing adjourned until 26 May.