A SCOT who has been imprisoned for a month in Barcelona after Brexit rendered him a “flight risk” is likely to face his 31st birthday still incarcerated – before his case comes to trial.

William Aitken, who has lived and worked in the Catalan capital for more than four years, was arrested during a protest supporting jailed rapper Pablo Hasel on February 17, the second night of demonstrations for the artist.

Seven people were arrested and six released the following day, but Aitken, 30, remained in what Spain calls “pre-trial detention” – the device used to incarcerate pro-independence politicians and activists for months before their trial over the 2017 independence referendum.

The problem is that although Aitken is a UK citizen, there is no extradition agreement between post-Brexit Britain and Spain. His lawyer David Aranda described this issue as a “barbarity”.

“The interlocutory statement is a legal barbarity, the judge must not know the UK’s exit agreement from the EU,” Aranda told the VilaWeb.cat news site.

“The Brexit agreement regulates a system of extraditions similar to that of European arrest warrants and allows the return of [accused] people in both directions, regardless of their nationality.”

Aranda is awaiting an appeal to the High Court of Barcelona, seeking to overturn the “flight risk” allegation.

Aitken went skateboarding with friends on the night he was arrested after leaving his partner, Fernanda Soler, at their home.

“I stayed home and watched a movie,” she said. “Since he was staying [out], I told him I would wait for him, but he didn’t come back.”

Police allowed Aitken to send her a voice message explaining that he had been arrested before he spent the night in a police cell.

“It just came to our attention then,” she said, “We watched as all the detainees from that night were released, except for William.”

Aitken faces public disorder charges, alleging that he attacked officers on the night of the protests, throwing stones at and damaging a van – the same charges faced by the other, freed accused.

Soler said her partner “has a job as head of department, indefinite contract, [is part of a] couple, a life built here”, and although letters from friends and supporters had maintained his spirits, he was worried about that his appeal could be unsuccessful.

The high court is expected to resolve this in the next two to three weeks, but March 26 is Aitken’s birthday and he will likely turn 31 in prison.

“That scares him a lot, obviously,” said Soler, who has managed to see him in Brians 1 jail.

“I hugged him for the first time since he left home a month ago and never came back.

“It’s all very strange, our life had been perfect so far.”