The parents of a Muslim convert who left his Oxford home for Syria and was suspected of joining Islamic State sent or tried to send him money when they had “every reason” to suspect it could be used to fund terrorism, a court has heard.

John Letts, 58, an organic farmer, and Sally Lane, 56, who worked in book publishing, allegedly ignored warnings about the behaviour of their son, Jack Letts, now 23. The couple, who are jointly charged under section 17 of the Terrorism Act 2000, deny three counts of sending or attempting to send a total of £1,723 to him between September 2015 and January 2016.

Alison Morgan QC, prosecuting, told an Old Bailey jury that the couple “knew or had reasonable cause to suspect” that the money might be used for terrorism, “because it was clear from the information available to them at that time that Jack Letts had joined Islamic State and was in Syria”.

Jack Letts had converted to Islam at age 16 and attended a mosque in Cowley Road, Oxford. Aged 18, he left for Jordan on what his parents appear to have believed was “a grand adventure”.

The jury heard he had become interested in Syria in 2014. One friend, who feared he had become radicalised and thought he was saying “worrying things”, knew of his plans to travel to Kuwait and urged John Letts to confiscate his son’s passport, Morgan said.

In May 2014 the couple permitted their son to travel to Jordan and Lane purchased a £400 return ticket.

Jack communicated with his parents by telephone, text and Facebook, Morgan said. In June he said he intended to take an Arabic course in Kuwait. When he missed his flight home, his father emailed him saying: “It’s weird you’re so far away but hey, you are on a grand adventure.”

Clear warning signs emerged in August 2014, the court heard. An Oxford friend of their son emailed John Letts telling him: “Apparently there is some concern regarding the company he [Jack] is keeping.”

By September 2014 the tone of communication changed dramatically. It was clear John Letts must have realised his son’s circumstances had changed, said Morgan.

In one email, Letts told his son: “A father should never live to see his son buried. Please, I beg you my son, come home, or at least leave where you are and do not get so involved.”

Five days later, Letts urged his son to make contact, telling him his mother was “collapsing with fear and sadness” and accused him of misleading them. He wrote: “You don’t have to die to help your fellow Muslims.”

In an email to a friend, Letts suggested his son had disappeared in Syria, Morgan said. She told jurors it was “inevitable that you will have sympathy for them as parents of a man who took himself to Syria against their wishes”.

The amounts of money were “not vast”, she said. “The defendants are not alleged to be terrorists. They have not been in trouble with the police before. It is not suggested that the defendants supported the ideology or actions of Islamic State, nor that they sent the money to provide positive support to Islamic State.”

However, they sent money “with knowledge or reasonable cause to suspect” it could fall into the hands of those who would use it for that purpose, Morgan said. The prosecution alleges they did it against the advice of a wide variety of people including friends of their son, academics they consulted, a charity worker who advised them on how to get him out of Syria, and several police officers.

Morgan said the law was focused on “the greater good” and it was not open to the defendants “to take the law into their own hands” and send money to their son, “whatever their own reasons and motives may have been”.

The case continues.