The Thai wife of a British journalist has been detained by police after her husband, an anti-monarchist, shared unflattering pictures of the country’s crown prince on social media and wrote about the royal succession.

Detectives said the photos shared by Andrew MacGregor Marshall, a former Bangkok-based Reuters correspondent who has been banned from the country since 2011 and now lives in Edinburgh, were fake and violated the kingdom’s royal defamation law.

His wife, Noppawan Bunluesilp, was visiting relatives in Bangkok, was detained by police and had electronic items seized. She was accompanied to the city’s crime suppression division by her father and three-year-old son and questioned before being released.

BREAKING—German paper Bild publishes extraordinary photos of Thai crown prince in Munich https://t.co/L9C7yWJGJe pic.twitter.com/zxDB67GAun — Andrew MacG Marshall (@zenjournalist) July 21, 2016

Marshall, who is the author of a banned book about the monarchy, told the Guardian that while he had criticised the royal family and Thailand’s draconian defamation law, he has always ensured Noppawan was never involved.

“I have always been very careful to protect my wife and her family from any consequences of my journalism,” he said via Skype from Hong Kong, where he is on business.

Thailand’s strict lèse-majesté laws provide for sentences of up to 15 years in jail for anyone found guilty of insulting, threatening, or defaming any of the leading members of the royal family.

According to Marshall, a Facebook post about an “imminent” royal succession and his sharing of a German tabloid’s striking photos, apparently of the crown prince, were probably “the trigger” for his wife’s detention.

According to Agence France-Presse, the commander of Thailand’s Central Investigation Bureau, Thitirat Nongharnpitak, told reporters the pictures were doctored, saying “the culprit is Andrew MacGregor Marshall who has violated lèse-majesté laws for several years.”

On Friday morning, the webpage remained blocked in Thailand.

Human rights observers believe that protection for free speech in Thailand has deteriorated rapidly since the military coup in 2014.

A recent UN review said lèse-majesté had been employed excessively to silence critics and curb free speech.

Last year, the New York Times found several critical articles on Thailand removed from international editions printed in the kingdom. And this week Thai subscribers to the Economist magazine were told by email that the latest issue, which features an article on the royal succession, would not be distributed owing to “the sensitive content … and the resulting potential risk”.

Marshall said this free speech crackdown was the reason he considered his wife and son’s return to the kingdom potentially problematic, yet the police raid still shocked him. “It’s one thing to be aware of [the risks], but to suddenly discover more than 20 police are raiding your wife’s family home is horrible.”