A former British soldier who travelled to Syria to fight against Isis faces up to 16 years in prison after being charged with terror offences in Turkey.

Joe Robinson, 24, was arrested on the beach while on holiday with his fiancée and her mother in the coastal resort of Didim last month.

Turkish media reported that security forces had found photos of Mr Robinson on social media showing him wearing military fatigues alongside members of the Kurdish People’s Protection Units (YPG) in Syria.

He told Turkish authorities he had received weapons training but did not take part in clashes during his three months providing medical assistance to the YPG, which Ankara classes as a terrorist group.

Mr Robinson’s solicitor, Kaya Sertkaya, said he has now been charged with membership of a terrorist organisation, which carries a maximum term of 16 years imprisonment.

Mr Sertkaya told The Guardian his client was being kept in a high security prison cell for 23 hours a day and having his phone calls monitored by authorities.

His girlfriend, law student Mira Rojkan, has been charged over sharing pro-Kurdish material online showing a separatist flag, Kurdish songs and fighters.

She was released on bail after six days but has been banned from leaving Turkey and is awaiting a court hearing.

US backed fighters make progress in driving Isis out of Raqqa

Ms Rojkan told The Independent the arrest as “awful”, adding: “They said someone had sent them an email saying we were terrorists about to do something in Turkey.

“They wouldn’t say who it was from. It is absolute nonsense. They arrested us on the beach while we were vacationing with my mother.”

The couple, originally from Leeds, are among several foreign nationals including journalists who have been charged with terror offences over alleged Kurdish links by Turkey.

The YPG is a major part of a coalition being armed and supported by the US-led coalition to drive Isis out of its de-facto capital of Raqqa, but Turkey has designated it a terror organisation because of its separatist ambitions.

Recep Tayyip Erdogan sent his forces into Syria last year to drive the YPG back from border areas and has publicly attacked fellow Nato and EU nations for supporting what he claims is a terrorist group.

Thousands of people have been arrested in Turkey over alleged connections with the YPG, insurgent Kurdistan Workers’ Party and factions that have claimed responsibility for recent terror attacks.

The crackdown is running alongside continuing purges on supporters of US-based cleric Fethullah Gulen’s “Hizmet” movement, which the Turkish government blames for an attempted coup last year.

Mr Robinson's family has sent a letter to Theresa May appealing for help and an online petition calling for his release has gained almost 3,500 signatures since being set up at the end of July.

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His father, Andrew Robinson, told The Telegraph he had received no reply from Downing Street after several weeks.

“We cannot let a young man, who did the right thing, go to prison. A man who made a change, who saved lives and defended our country's interests,” the letter said.

“To do anything other than provide effective and immediate support to a man who is a British citizen, a veteran and a hero, is to show a deep disregard for your veteran forces, and to ally yourselves with an oppressive state.”

Mr Robinson, originally from Accrington in Lancashire, served with the Duke of Lancaster’s Regiment in Afghanistan in 2012 and said he decided to join the fight against Isis after a militant killed 39 British tourists on a Tunisian beach in June 2015.

After returning to the UK in November that year he was arrested on suspicion of terror offences at Manchester Airport but released from bail 10 months later.

Britain does not class the YPG as a terrorist organisation but has investigated anyone found to have joined the group and charged some with terror offences.

Relatives believe British officials have so far been unable to visit Mr Robinson in jail to offer support and check the conditions of his imprisonment.