David Fulton, 60, a former British Army major, and his wife, Fiona, 46, are being held in separate prisons.
Mr Fulton, originally from Troon, Scotland, is being held in a cell at the high security Mile Two prison just outside the capital, Banjul.
His wife and the couple’s adopted daughter, Elizabeth, aged two, have been detained at a police station in Banjul.
A family friend has said that Mrs Fulton, originally from Torquay, Devon, is being treated well, with police officers volunteering to go on errands to buy nappies.
However, there are concerns about her husband’s safety in a jail built in colonial times and noted for its “tough” regime.
According to some reports in Gambia, other prisoners held on similar charges have been poisoned.
The family friend, who has asked not to be identified for fear of being targeted by the Gambian authorities, said: “Fiona has been treated well, but we are not sure about David. We don’t think he’s fared quite as well. He’s not eating.”
The Fultons, who moved to Gambia 12 years ago to work as missionaries, have two other children, Iona, 20, and Luke, 17, who are believed to live in the Exeter arera.
Mr Fulton works as a chaplain in the Gambian Army and his wife looks after the terminally-ill and spends time visiting women in their homes and in hospital.
They were arrested last Saturday after police arrived at their house in the village of Kerr Sering, an hour’s drive from the capital.
Yesterday they appeared in court in Banjul to face charges of sedition. They were offered bail, but have so far been unable to meet the requirement of four Gambian property owners to vouch for them.
Although the country is constitutionally secular, the population is 90 per cent Muslim.
The friend said: “While we are free to speak out, in Gambia you cannot.
“As a chaplain part of David’s job is to provide comfort to all sorts of people: people high up, people low down, and people who have perhaps fallen out of favour.
“I don’t know of anything they have done that could be called sedition.
“Their whole focus has been teaching the gospel of Jesus Christ.”
The friend went on: “Sedition is a serious crime anywhere and the penalty would not be light.
“Obviously we are concerned. A lot of Christian people all over Britain and all over Europe are praying for them. We are just trusting the outcome will be good.”
Pastor Martin Speed, of Westhoughton Pentecostal Church in Bolton, Greater Manchester, said the couple had visited Christians at his church to talk about their work.
He emphasised that Mr Fulton’s work was not political, but said: “There does seem to be a growing difficulty of Christians in Gambia.”
The Foreign Office confirmed the arrests and said consular staff had been in touch with the couple.
I have visited Banjul, Gambia several times. While the people are friendly to westerners, they are hostile to the Gospel. Please pray for the Fulton family in this difficult time and that God will get all the glory.
Immanuel Southern Baptist Church 