An Oxford couple have been given suspended jail sentences after being found guilty of sending money to their son while he was living under Islamic State in Syria that could have been used to fund terrorism.

John Letts, 58, an internationally renowned organic grain farmer, and Sally Lane, 57, a former Oxfam fundraiser, defied police warnings and sent £223 to Jack Letts, now 23, despite concerns he had joined Isis.

They knew they were breaking the law, the Old Bailey jury was told. In one message, Lane told her son: “I would go to prison for you if I thought it gave you a better chance of actually reaching your 25th birthday.”

The couple refused to believe that Jack, a Muslim convert, had become an extremist, and sent the money because they believed he was in mortal danger. The prosecution argued that the couple “did not want to hear the truth” and “turned a blind eye to the obvious”.

The judge Nicholas Hilliard QC sentenced the couple to 15 months’ imprisonment, suspended for 12 months. He said that in their love for their son they had lost “sight of realities” even though “the warning signs were there”.

Jack Letts in Raqqa, Syria. Photograph: Counter Terrorism Policing South East/PA

The jury was not told that Jack was believed to be in the custody of Kurdish authorities, accused of being a member of Isis. In an ITV interview from prison in February, Jack, who has dual UK and Canadian citizenship, said he doubted officials from either country would help him because “no one really cares”.

In a statement outside court, the couple said: “We have been convicted for doing what any parents would do if their child was in danger.” There was no suggestion during the trial that the money they sent was used for terrorism, they said, adding that they felt badly let down by the police and the government.

They said they had pleaded with the government to help get Jack back to the UK. “Having escaped from Isis, he is in limbo,” they said. “The heavy price we paid today is an indicator of the love we have for our children. We are committed to help Jack return home.”

His parents initially sent him £223 in September 2015. They then attempted to send another £1,000 in December, claiming Jack, who has obsessive compulsive disorder, was trapped in Raqqa and needed the money to escape. Those transfers were blocked.

The jury deliberated for almost 20 hours before finding them guilty of the September charge but not guilty of the December charge. They could not reach a verdict on a third charge, which will remain on file.

Jack converted to Islam aged 16 and travelled to Syria in 2014, when he was 18. The couple paid for his return flight to Jordan in May 2014, believing he was on a “grand adventure” learning Arabic, despite one of Jack’s friends warning them about his growing extremism.

Jack then moved to Kuwait and married in Iraq, before he rang his mother to say he was in Syria in September 2014.

Wanting to help Jack was not a defence, the prosecutor Alison Morgan QC told the jury. “Once Jack Letts made the decision that he did to follow Islamic State to Syria and live in Islamic State territory, and to defend its ideals and goals, the normal parenting rules for what you can do to help your child change.”

Jurors heard that Jack said “the most appalling things” on social media and in messages to his parents. In one Facebook comment, underneath a photograph posted by a former schoolfriend with young British army comrades, he wrote that he would like to “perform a martyrdom operation in this scene”. He said he wanted to behead the same schoolfriend, and warned British police who had raided his parents’ home to “die in your rage soon you’ll be the one being raided”.

His mother, in evidence, denied her son posed a danger, dismissing his comments as “playing to the gallery”. The couple claimed Jack was in immediate mortal danger because he wanted to leave and the money was to get him out of Syria. “That was the price we had to pay, and if that was the price we had to pay then we were going to do it,” Lane told the court.

His parents have made repeated attempts to get the UK and the Canadian governments to help get Jack out of Syria, including organising protests and staging hunger strikes to draw attention to their case.

DCS Kath Barnes said investigators had “huge empathy” with the couple but it was not for them to choose when it was “OK to break the law”.

Jenny Hopkins, of the Crown Prosecution Service, said it was natural for parents to care for their son, but the couple had been warned “multiple times”. “They chose to ignore that advice and at one point set up a fake identity to send him money,” she said.

• This article was amended on 24 June 2019 to change text in the first paragraph that did not conform to the Guardian’s style guidelines.