Video report by ITV News Security Editor Rohit Kachroo

He's missing his mum's cooking, hoping his sister's exams go well and even bringing news of a forthcoming grandchild.

The letters of British-born Ashraf Mahmud Islam to his parents are often typical of any young person living away from their family, describing himself as their "big baby boy".

Except, he's writing from a Kurdish prison in northern Syria.

Because he's a captured member of Isis.

Ashraf Mahmud Islam has been imprisoned in northern Syria since January. Credit: ITV News

Ashraf has been held by the YPG forces for the last 10 months - but it appears not indefinitely.

ITV News has learned his Kurdish captors are unwilling to continue to house him and his fellow captives or put them on trial. They want to move them on.

Yet actions have already been taken in Britain to stop Ashraf returning.

ITV News has seen rare evidence of the Home Office's attempt to end his British citizenship - and his family's attempt to fight the decision.

Our investigation began when our producer Zana Omer gained access to interview Ashraf from inside prison, where he talked about why he joined Isis and why he wants to "come home".

What did he say about joining Isis?

Ashraf said he had travelled to Syria in 2015 to "help the Syrian people". He didn't confirm his starting location but said he reached Syria through Turkey and eventually made it to Raqqa.

Ashraf claimed he joined Isis because they were one of the few groups that would "accept foreigners" and had mixed with British fighters, including Jihadi Jack. He said he hadn't met Jihadi John.

He didn't discuss his combat role and ITV News cannot confirm the extent of his actions during his time in Syria.

He claims he later lost heart and, after moving to the east of Deir Ezzor, says he surrendered to the YPG.

Ashraf Mahmud Islam says he now regrets joining so-called Islamic State. Credit: ITV News

"I was trying to leave because I realised what I did was a mistake and I want to fix that mistake and I've been in prison ever since," he said.

He also said "most British Isis fighters" have been killed and added that "most" who were still alive "want to leave too".

But he added: "The issue with this is they’re not able to cause the UK is very strict with its laws against terrorism."

This is when he talked about his own hope of returning "home".

The questions surrounding Ashraf Mahmud Islam are not moral ones, but legal ones, explains ITV News Security Editor Rohit Kachroo

Did he confirm where in Britain he was from?

Ashraf Mahmud Islam confirmed he was born and raised in Britain. Credit: ITV News

He revealed parts of his private life, talking about his loving parents and his belief that he had himself become a father to a son while in prison.

Yet Ashraf gave only a few details on his connections to the UK.

He confirmed his full name, that he was "born and raised" in Britain and his "whole family" still lives there, although he didn't say where.

While his accent suggested he had lived abroad, he said he only has British citizenship and added: "This is where my heart lies."

He later refused our follow-up attempts for more details on his age or birthplace.

So we began our own investigation into his UK links.

So who exactly is he?

The Facebook page with an image resembling Ashraf Mahmud Islam. Credit: Facebook

A search for Ashraf Islams in Britain gave us too many options. We had to narrow it down.

So we took to social media to see if we could find a profile that matched.

We found one with the right name on Facebook and a photograph that at least looked similar.

To try to see if was the right man we reached out to his few Facebook friends.

We showed them a photograph of Ashraf’s interview and they confirmed it was our man.

What did they say about him?

Friends of Ashraf Mahmud Islam's confirmed he had studied law with them. Credit: ITV News

They had studied A Levels with him at the Nottingham Law Academy, an English education institute in Dhaka, Bangladesh. He had told them he was from London and his father was a lawyer.

"He suddenly vanished ... during the Isis epidemic," one contact told ITV News.

A few of them reached out to his family after he left, but they said even they didn't know where he had gone.

The young men's ages and responses gave us enough information to refine our search at the UK end and soon we had a possible birth certificate match.

What did it show?

A birth certificate confirmed the birth of a Ashraf Mahmud Islam in the London borough of Tower Hamlets in 1996. Credit: ITV News

The date of birth was 1996, which would make him 18 at the time he was supposed to have studied law in Dhaka.

And his father’s job is listed as a "legal adviser"; his friends had told us his dad was now a lawyer.

We looked up the listed address and it appeared the family had left the property in the last 10 years.

We formed a working theory that perhaps they had moved to Bangladesh while Ashraf was in his early teens.

Next we wanted to make contact with his family.

His father’s full name on the certificate and law connections made him far easier to find. We soon traced him to a law firm in Dhaka, where he is listed as a barrister.

His profile picture even bore similarities to his son.

So we called him and finally got our confirmation, via his solicitor - plus a lot more: he also sent across Ashraf's letters.

What do the letters say?

Ashraf Mahmud Islam wrote home about his imprisonment as his father's 'big baby boy'. Credit: ITV News

Ashraf's correspondence with his family began a few months after he was jailed in Kurdistan in January - and he appears remorseful and homesick.

"I'm so sorry for all the pain I gave you," he writes to his mother in a letter in which he signs off "love you loads".

In a separate reply to his father, he hopes his sister "does good" in her GCSEs, adding: "Hopefully better than me."

He expresses remorse in his letter to his father. Credit: ITV News

Addressing his plight he says, "I know everything you told me was right and I realise that now. If I could go back then I would have listened to you and obeyed."

Then in the margin, almost as an afterthought, he writes: "PS. I think you became a grandfather but have no news yet."

Why did the family share the letters?

The Home Office letter to Ashraf Mahmud Islam, signed by the then home secretary Amber Rudd. Credit: ITV News

They were part of a bundle of papers which formed the family's appeal of Britain's decision to block Ashraf from returning to the UK.

We also obtained the original letter issued by the Home Office, signed by the then home secretary Amber Rudd, which issued the order to deprive Ashraf of his British citizenship.

"The reason for the decision is that is it assessed that you are a British/Bangladeshi dual national who has travelled to Syria and is aligned with ISIL," Ms Rudd wrote.

"It is assessed that your return to the UK would present a risk to the national security of the United Kingdom."

The letter added: "I am satisfied that such an order will not make you stateless."

It was issued at some point before Ms Rudd was replaced by Sajid Javid as Home Secretary at the end of April.

How do Ashraf's family dispute it?

The family's legal representatives claim the order will make Ashraf stateless as he will "not be entitled to Bangladesh nationality by descent" or gain citizenship.

They also said he could not fairly contest the order because he remains in detention and want him to be "given leave to enter the UK to prosecute his appeal".

Why might Ashraf not be held for much longer?

Dr Abdulkarim Omar of the foreign relations commission in North Syria. Credit: ITV News

The Kurds holding him confirmed to ITV News their intention to move on the Isis captives.

"We will not prosecute them on our land and we will not keep them in our areas," confirmed Dr Abdulkarim Omar of the foreign relations commission in North Syria.

He suggested other countries should assume the duty of care, saying: "We will try, through our contacts and through meetings, to make sure everyone takes responsibility."

And if that is not forthcoming?

"When we lose hope, we will have a decision to make and this decision we will announce it in the right time," Dr Omar confirmed to ITV News.

"I want to emphasise again that we will not put them on trial and we will not keep them here."

What did Ashraf say about returning to Britain?

Ashraf Mahmud Islam told ITV News he will willingly be imprisoned in the UK. Credit: ITV News

In his interview with ITV News, he insisted he is not an enemy of his home state.

"I never made any videos against the UK, I never did anything against the UK," he said.

"I realise I made a mistake and I want to go back."

The barrister's son said he would be happy to continue to be imprisoned, as long as it was on British soil.

If he was given "a choice between staying here or going back to the UK and doing prison time" he said: "I'll go back to the UK any day."

And did he have any indication what would happen now?

"I’ve no news of what’s going to happen to me, what’s going to be the outcome of the situation I’m in," he said.

"So far, all I’ve been thinking about is just going back. That’s all I want."

In a separate statement, a family member told ITV News they "cannot understand" what made Ashraf "brainwashed to the extent to do what he did" and join Isis.

They added: "All we want is Ashraf to come back home safely."

What has the Home Office said about the situation?

A Home Office spokesperson told ITV News: "Any British Citizen may be deprived of his or her citizenship if the Secretary of State is satisfied that it would be conducive to the public good, for example where they pose a threat to national security, unless to deprive would render them stateless."