The mother and father of Muslim convert dubbed 'Jihadi Jack' said the British government has left him "to rot" as they desperately seek his release from Syrian cell.

Jack Letts, from Oxford, has now been charged with being a member of Isis, according to officials from the Kurdish region of Syria who spoke to the BBC.

A statement from Democratic Federation of Northern Syria (DFNS) says Letts had been taken to a prison in Qamishli, Rojava, northern Syria. His case is being investigated by local police. This marks the first Kurdish forces confirmation that Letts is a prisoner of war.

Parents John Letts, 56, and Sally Lane, 54, began a hunger strike outside St Paul's Cathedral in central London last Thursday (19 October) in protest over what they claim is Parliament's failure to help bring their son home.

Their son Jack Letts was labelled 'Jihadi Jack' after travelling to Isis-controlled territory in 2014, and has been accused by the UK government of fighting for Muslim terror group Isis.

The 21-year-old made his way into Kurdish territory in May this year and is being held in a cell by the anti-Isis Kurdish YPG militia.

His Oxford-based parents have not heard from him in three months.

But in a novel twist, Lane, a book editor said that her son was actually working as a double agent against Isis.

She said: "Him and a group of friends were working against Isis from the inside.

"And in fact I've spoken to some of his friends and they are still in hiding. They say things were made very, very difficult for them when Jack was labelled a terrorist by the British Government because it then compromised their position."

Lane said the position their son has been left in by the government is "immoral and illegal". She said it was wrong to "leave somebody in a hole with no trial, no process, no charge."

She told Russian broadcaster RT: "At the moment he's been locked in a room. They've thrown away the key, and he's just been left to rot."

Previously, Letts, an organic farmer and baker, said: "The British government has a duty to protect its citizens. We believe the government's policy of preventing anyone who went to Syria from returning to the UK is short-sighted and ultimately counter-productive."

But earlier this week Foreign Office minister Rory Stewart told the BBC that the government's response to the vast majority of British fighters for Isis would be to "kill them."

He said: "These are people who have essentially moved away from any kind of allegiance towards the British government.

The former diplomat and infantry officer added: "They are absolutely dedicated, as members of the Islamic State, towards the creation of a caliphate; they believe in an extremely hateful doctrine which involves killing themselves, killing others and trying to use violence and brutality to create an 8th-century, or 7th-century, state so I'm afraid we have to be serious about the fact these people are a serious danger to us, and unfortunately the only way of dealing with them will be, in almost every case, to kill them."

This follows a warning two weeks ago from defence secretary Sir Michael Fallon who said that Britons joining Isis had made themselves legitimate targets for RAF or American missiles.

A Foreign Office spokesman added: "As all UK consular services are suspended in Syria, it is extremely difficult to confirm the whereabouts and status of British nationals there. Anyone who does travel to Syria, for whatever reason, is putting themselves in considerable danger."