A student accused of travelling to Syria to join rebel forces had an horrific video on his mobile phone of a young man having his throat cut, a court has heard.

David Souaan, 20, was arrested at Heathrow airport in May before getting on a flight to Serbia, where his family home was.

When police seized his laptop and iPhone they found a mass of pictures, videos and documents revealing his “extremist sympathies” and that he had not only been fighting in Syria before but was intending to return, the Old Bailey heard.

The prosecutor, Sarah Whitehouse QC, said one video clip on his iPhone was “so graphic and so shocking” that it could not be played in court.

She told jurors: “An officer will describe it to you. It shows a young looking man kneeling while his head is held back and another man cuts his throat.”

The son of a Serbian Christian mother and a Syrian Muslim father, Souaan had come to the UK on a six-month visitor’s visa in 2012.

In 2013, the devout Muslim was given a three-year student visa and began studying global politics and international relations at Birkbeck College in London, living in halls of residence in Malet Street.

Fellow students described him as a “nice, softly spoken young man” but, the court heard, by spring 2014 he had changed and had become “harder and more serious when anyone mentioned anything to do with girls and alcohol”.

After he began talking about radical Islam and showing off pictures of himself in combat gear and a gun, some friends told police, the court heard.

Whitehouse said it may be suggested in court that the friends had misunderstood, misheard or exaggerated what they had seen but evidence would show that he repeated his views in texts and Skype messages.

She went on to describe a video clip created on 20 December 2013, which showed Souaan and four others walking through a derelict urban street carrying automatic weapons.

Souaan had apparently filmed himself attending a demonstration in the UK too.

In the background a man can be heard to say: “The flag of tawhid in London, all praise be to Allah, lord of the universe” – in reference to the fervent wish of Muslim extremists to see the black flag fly over Downing Street.

On 3 January this year, Souaan sent a text message to a friend saying: “Hello from Syria” accompanied by an image of himself sitting on a destroyed building. Among the pictures, there was an image of Souaan holding the Islamist flag and pictures of him from Syria with weapons in his hands.

Several documents were also found on Souaan’s computer, including a form which he had filled in to excuse himself from handing in course work in December 2013.

Jurors were told that on the “mitigating circumstance claim form”, Souaan had said he was behind in his college work because the family home in Syria had been hit by rockets launched by the Assad regime and that his grandfather and cousins had escaped to Turkey.

He wrote that he had flown to Turkey to “give them support” but there was no mention of him being in Syria, the court heard.

Souaan denies preparing for terrorist acts in Syria around the time he was stopped by police on 31 May this year.

The Old Bailey trial continues.