The parents of a Muslim convert suspected of joining Islamic State sent or attempted to send him money despite him telling them he wanted to behead a former schoolfriend who had joined the army, a jury has heard.

John Letts and Sally Lane challenged their son Jack Letts after he responded to a Facebook post by Linus Doubtfire in 2015, the Old Bailey heard.

Doubtfire had posted a picture of a group of soldiers upon his completion of a commando artillery course, to which Jack had responded: “I would love to perform a martyrdom operation in this scene.”

The jury heard that when the parents questioned him after his post, Jack said: “I admit it was wrong if I seemed like I was joking. I genuinely believe attacking the British army is a very praiseworthy action … I would happily kill each and every one of Linus Unit personally.”

The court was told that Jack continued: “This message is for you, Mum and Tyler [his younger brother], I honestly want to cut Linus [sic] head off. I hope he finds himself lost in Baji or Fallujah one day and sees me whilst I’m armed and I’ll put six bullets in his head.”

His mother responded by asking for proof it was him writing the messages about beheading, the jury heard. The prosecution said this demonstrated that the couple had doubts about whether they were dealing with Jack before they agreed to send money.

John Letts, 58, and Lane, 56, from Oxford, are accused of sending or attempting to send their son a total of £1,723 between September 2015 and January 2016 after he travelled to Syria. They deny three charges under section 17 of the Terrorism Act 2000.

The prosecution claims that from information available to them at them time, they “knew or had reasonable cause to suspect” the money might be used to fund terrorism.

Jack converted to Islam aged 16 and left for the Middle East aged 18. He married in Iraq before going to Syria. He is now 23.

The jury heard that in another Facebook post, seen by his parents, Jack shared a picture of himself at the Tabqa Dam in Raqqa. Alison Morgan QC, prosecuting, said Jack could be seen “making a gesture with his finger”.

Email, text and Facebook messages later retrieved by police showed his parents’ shock and despair on discovering Jack’s whereabouts, the court heard. Lane told one of Jack’s friends he had phoned her and was “OK for now” but “he’s in the worst place possible and doesn’t want to leave”.

She confided to another friend: “I still can’t quite believe he’s been so utterly stupid. I can only think that his mental illness (OCD) was deeper than we thought.”

John Letts told his son when he showed no sign of returning to the UK: “You are now caught up in a crusade that has left so many lives shattered.” He said Jack was “a pawn … helping to spread hatred, pain, anger, suffering and violence”.

Police began investigating Jack’s activities in early 2015 and warned his parents they could not send him money, the court has heard. When they told their son of this, he responded: “Please convey to the British police that I’m not planning on coming back to their broken country … Convey to them from me: ‘Die in your rage, soon you’ll be the ones being raided.’”

In exchanges with her son, Lane asked him to stop telling her of his “barbaric impulses”.

In one, she blamed herself, telling Jack: “Clearly I indulged you, I made you think you were the centre of the universe. I was a terrible parent that gave you too much power as a child. I should have made you adapt to the world, instead of adapting myself to your world. I have done you no favours by doing this,” the jury heard. She continued: “This is a sign of mental illness and I have to bear some responsibility for that as your mother.”

On 2 September 2015, Lane transferred £223 to an unknown person in Lebanon at Jack’s request, it was alleged. She asked her son: “Can you guarantee they have nothing to do with Jihad? As I could go to prison.” Jack said they did not, and she added: “As I said to you before, I would go to prison for you if I thought it gave you a better chance of actually reaching your 25th birthday,” the jury heard.

When police questioned her about the transfer, she said she hoped the money would help Jack to get glasses, and she did not believe she was supporting terrorism. She told officers: “I am trying to get him out of terrorism and I don’t think he’s a terrorist,” the jury heard.

The prosecution alleged the couple had no idea what was going on in Jack’s world, and had accused him of lying to them.

A second attempt, to transfer £1,000 to Lebanon, was made in December 2015 when Jack said he wanted to leave Syria and needed it to pay for travel and smugglers, the jury heard. When that was blocked, a third attempt, to transfer £500 using a false name to Lebanon, was made on 4 January 2016. It was also blocked, and the couple arrested the following day.

Lane, in an email to a work colleague, said the couple had gone ahead despite the warnings from police they risked prosecution. They had had to “weigh up the pros and cons of sending him the money so he can get out alive and not sending him the money so we wouldn’t get a criminal record,” she wrote.

“We believe most parents would make the same decision we did,” she added.

Morgan, concluding her opening, said it was clear the couple knew they were breaking the law. She added: “They no longer knew the person that their son had become. Someone who was quite happy to put six bullets into the head of a former schoolfriend.”

The case continues.