A British teenager who made a graveside pledge to devote herself to the PKK cause has been convicted of intending to join the proscribed Kurdish terrorist organisation to fight Islamic State.

Silhan Özçelik, 18, from north London, ran away from home, took a train to Brussels, and left behind letters and a video for her distraught family telling them she wanted to be a guerrilla fighter and was joining the Kurdistan Workers’ party’s women’s militia.

She is the first British citizen to be convicted for trying to join the campaign against Isis jihadis in Syria.

Özçelik, who was 17 when she went to Belgium in October 2014, had been “smitten” by the PKK since she was 13 after watching a film, Comrade Beritan, about a PKK female guerrilla who threw herself off a cliff rather than face capture and died in 1992.

She had also visited the Turkish grave of Leyla Saylemez, whose nom de guerre was Comrade Ronahi and one of three female PKK activists shot dead at a community centre in Paris in January 2013.

In the 25-minute video Özçelik left behind explaining her decision to her family, she said she had taken soil from Ronahi’s grave and made a promise, which she was now going to fulfil.

The jury at the Old Bailey dismissed Özçelik’s claim that she had invented the PKK story because she was running away to meet a 28-year-old man in Belgium with whom she hoped to kindle a romantic relationship, and wanted to spare her family shame in the strict, traditional Kurdish community.

Dan Pawson-Pounds, prosecuting, said the video and letters, in which she passionately described her love for the PKK, her wish to become a militant and “bride to the mountains”, and her desire for her family to be proud of her, “couldn’t be clearer or more consistent” with her long-held ambition to be a fighter and guerrilla.

Özçelik had made the incriminating video because “she had no intention of ever returning to the UK”, the jury heard.

The court was told she had yearned for five years to join the PKK, her video explaining that she had tried to join previously, contacting someone “high ranking in the organisation” when she was 16, but being rejected because she was too young. She tried again at 17, but was once more rejected on the grounds of her “emotional decision”.

She wrote to her family: “I have been thinking about this for many years. I have been wanting to join for many years. Furthermore I am leaving my diary, and it is written in my diary … I wrote, every minute, how much I wanted to become a militant, how much I wanted to become a guerrilla, are all written in those pages.”

Özçelik was “passionately engaged” with the PKK cause. She spoke of her anger that Islamic State at that time was crushing her people in Kobani, the largely Kurdish city in Syria, and that no men were going out there to fight against Isis. She was attracted by the active role women were allowed to play in the PKK, the jury was told.

She wrote: “Maybe I will go to Kobani, or I will not go. That is a different matter. It is up to the PKK to decide. But I see myself as a fighter, I see myself as a militant, a guerrilla.”

Özçelik was the baby of her family. She was 10 years younger than the youngest of her three siblings and found life in the strictly traditional family home restrictive. Her father, a chef, and mother, a textile factory machinist, gained political asylum in 1993 and settled in Britain. Though born in London, Özçelik identified strongly with her Kurdish roots and told school friends she used the name Dersim, the Kurdish name for the city of Tunceli – where the family of Comrade Beritan, the nom de guerre of Gülnaz Karataş, was from.

Posters, collages and Photoshopped pictures of PKK slogans and armed female guerrilla fighters were found in her bedroom. She had “glorified” the fighting and use of guns, especially by women, the prosecution said.

At the time she ran away she was a student of media studies at Holloway College. She had gained nine GSCEs at secondary school but then dropped out of her previous sixth form, where staff described her as “insular”, with few friends and seemingly “a bit depressed”.

She had lied to friends about her reasons for going to Belgium, telling them variously that she was going to visit an uncle, to study in Germany, to go travelling, or to escape a jealous ex-boyfriend, the jury heard.

There was no evidence Özçelik had joined the PKK, made contact with PKK members or travelled to Turkey or Syria before she returned to Britain from Cologne in Germany in January 2015 and was arrested at Stansted airport.

The jurors were told they had to be sure of two things: that she had the intention to commit acts of terrorism by joining the PKK, and that she engaged in conduct in preparation for the act. That preparation, said the prosecution, involved buying a one-way ticket to Brussels, recording a 25-minute video to her family and writing two letters to them saying she was joining the PKK and giving her reasons why, and by getting on the train to Belgium.

Before leaving, Özçelik had wiped most of the data from her mobile phone, which she left behind. She told her family she would be in contact, but that when she did they could not mention her name on the phone. This, the prosecution said, was consistent with her intending to join a terrorist organisation and knowing the authorities might be monitoring her communications.

Özçelik told the court she thought making the video was “cool” and she made it so that if things did not work out with the man in Belgium – and they did not – she would still be accepted back by her family. If they thought she had been fighting for the PKK, that was more heroic than the shame of knowing she had gone to meet a man, she said.

The jury of nine women and three men took five hours to reach a unanimous verdict.

Sentencing her to 21 months in a young offender institution, the judge, John Bevan, described her as “a stupid, feckless and deeply dishonest young woman”. Although there was no evidence she did anything to “advance the aim expressed in the video”, he told her she was “immature” and the “author of your own misfortune”.

Dismissing her counsel’s appeal for a suspended sentence, the judge added “any conviction for an offence of terrorism is serious”. He was not satisfied that the PKK was her only motive for travelling, and her emotions for Mehmet, the man in Belgium she said she hoped to have a relationship with, may have been a part, but the jury had decided “for a time at least”, joining the PKK was her intention.

He said her sentence was much reduced “because of the highly unusual factors of this case”. She would have to live with the “long lasting consequences of a conviction for terrorism”, he added .

As he passed sentence, sobs could be heard from the public gallery where members of her family had sat throughout the trial.

After the trial Commander Richard Walton, head of the Met’s counter-terrorism command (SO15), said: “We continue to remain concerned about the number of young women and girls being drawn into all forms of terrorism.

“We urge parents and families to talk to us at the earliest opportunity if they have concerns about any girl or women being enticed into supporting terrorist groups like the PKK or Isis.”