A British teenage girl who ran away to join the fight against Isis militants has been jailed for 21 months.

Silhan Ozcelik, 18, is the first Briton to be convicted of trying to join the banned Kurdish group PKK.

She was on trial at the Old Bailey charged with engaging in conduct in preparation for terrorist acts.

The court heard she left Britain on the Eurostar in October 2014 and was arrested in January after getting off a flight from Germany at Stansted airport.

After she left, she wrote letters and made a video telling her family she had gone off to try to fight with the Kurdistan Workers Party, the court heard.

But in her defence she claimed that was a lie and she actually went to see a man she had formed a romantic attachment to in Brussels.

The jury deliberated for less than a day to find her guilty and she was sentenced to 21 months in prison by Judge John Bevan QC.

The PKK hangs as armed kurdish militants man a barricade, on November 18, 2015 in the Sur district of Diyarbakir (AFP)

Sentencing, Judge Bevan told Ozcelik: "You are a stupid, feckless and deeply dishonest young woman. You have lied to your family and this jury."

The trial had heard the defendant travelled to Brussels on a one-way Eurostar ticket four months after dropping out of college.

She left behind a letter for her family, informing them she had joined the ranks of the PKK - a proscribed terrorist organisation under UK law.

In it, she wrote: "Believe me this is the right thing for me to do. I am so happy right now that I have become a militant."

In a 25-minute video in Turkish, she told her family: "My fight, my struggle is not just for the Kurdish people, it is for all people, for all women.

"It must not be misconstrued. This is not a Kurdistan matter. Even if Kurdistan is established today I will not return."

Soon after her disappearance, her 27-year-old brother Engin told police he had noticed his sister had begun to take more care of her appearance - wearing make-up, styling her hair and watching her diet.

He suspected she may have had a romantic interest, jurors were told.

Ozcelik, who is of Turkish descent, made no comment to police after she was arrested at Stansted Airport in January.

PKK soldiers stand to attention after arriving in the northern Iraqi city of Dohuk (Getty)

Giving evidence, she denied that she had joined PKK saying she had lied about the real reason for her trip because going off with a boy was "really shameful in our community".

She told jurors: "In the Kurdish community, if you say I'm joining the PKK, everyone will look up to you, they'll respect you. If you say you're going off with a boy, the situation changes."

While overseas, Ozcelik visited Holland and Germany with a 28-year-old man called Mehmet, with whom she wanted a relationship, the court heard.

She told the court she initially "felt like an adult" but then ended up doing the cooking and cleaning and felt oppressed, much like she was in London.

She called her concerned mother who told her to come home after she told her the truth, Ozcelik said.

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The defendant said she had a long standing interest in the PKK since she watched a film called Beritan as a 13-year-old.

The film, which depicts the death of a female fighter who threw herself off a cliff to avoid capture, was "melodramatic and in some ways romantic", she said.

Ozcelik later made a collage of PKK leaders, Kurdish and Turkish socialists, and Che Guevara as a school project.

From 2013, she was keenly observing political developments involving the PKK, including a push from its leader for peace.

The defendant also followed events in Kobani, in Syria, during Islamic State massacres.

She said: "The only people defending them over there was the YPG, the PKK. It was amazing, the fact that they were there and they were trying to protect innocent people - I just admired it."

She told the court she felt "cool" recording the video on her laptop with the aid of notes, but it seemed funny later.

Asked if she regretted it now, Ozcelik, of Highbury Quadrant, Holloway in north London, replied: "Yeah I regret everything - it's caused trouble for my family and everyone else."