Joe Gordon was in Colorado when he blogged excerpts from banned biography but was arrested when he visited Thailand

A court in Thailand has sentenced a Thai-born American to two-and-a-half years in prison for defaming the country's royal family by translating excerpts of a locally banned biography of the king and posting them online.

Joe Gordon, 55, stood calmly with his ankles shackled in an orange prison uniform as the sentence was read out in a Bangkok criminal court.

Judge Tawan Rodcharoen said the punishment initially was set at five years behind bars but he reduced it because Gordon pleaded guilty.

King Bhumibol Adulyadej of Thailand, which has the world's toughest laws on lèse-majesté. Photograph: Rungroj Yongrit/EPA

Gordon posted links to the banned biography of King Bhumibol Adulyadej several years ago while living in Colorado in the US. The case has raised questions about the applicability of Thai law to acts committed by foreigners outside Thailand.

Speaking after the verdict, Gordon said: "I am an American citizen and what happened was in America."

He said he had no expectation of being let off easy. "This is just the system in Thailand," he said. Speaking later in Thai, he added: "In Thailand they put people in prison even if they don't have proof."

Gordon was detained in late May during a visit to Thailand, where he had returned for medical treatment. After being repeatedly denied bail he pleaded guilty in October in hopes of obtaining a lenient sentence.

Thailand's lèse-majesté laws are the harshest in the world. People found guilty of insulting the king, queen or heir to the throne face three to 15 years behind bars. The nation's 2007 Computer Crimes Act contains provisions that have enabled prosecutors to increase sentences.

Thailand is under pressure both at home and abroad to amend the laws, which critics say are too harsh and have been used for political harassment.

The US consul general in Bangkok, Elizabth Pratt, told reporters after the ruling that Washington considered the punishment "severe because he has been sentenced for his right to freedom of expression".

New York-based Human Rights Watch has urged authorities to change the laws, saying the penalties being meted out are "shocking". In November, 61-year-old Amphon Tangnoppakul was given a 20-year jail sentence for sending four text messages to a secretary of the former prime minister, Abhisit Vejjajiva. The content of the messages was deemed insulting to the monarchy.

The Thai information minister, Anudith Nakornthap, has warned that Facebook users who click the "share" or "like" button on content that insults the Thai monarchy are committing a crime. Anudith said Thai authorities had asked Facebook to remove 86,000 such items between August and November.

Gordon, a former car salesman, is accused of having translated excerpts from the unauthorised biography The King Never Smiles, published by Yale University Press, into the Thai language and publishing them in a blog. He provided links to the translation to other two web forums, prosecutors said.

In the banned book, author Paul M Handley retraces the king's life, alleging he has been a major stumbling block to the progress of democracy in Thailand by consolidating royal power over his long reign.

Bhumibol, the world's longest-reigning monarch, is profoundly revered in Thailand and is widely seen as a stabilising force. Monday was his 84th birthday and he called on the country to unite in response to the worst floods in more than half a century.

The king is frail and has stayed at a Bangkok hospital for more than two years.