Academic and political columnist Pramote Nakhonthap has lost his final appeal against a suspended prison term for defaming former prime minister Thaksin Shinawatra over the alleged "Finland" plot to topple the constitutional monarchy.
In a ruling read on Wednesday, the Supreme Court upheld the judgements of the two lower courts.
In 2009, the Criminal Court convicted and sentenced Pramote to a one-year jail term, suspended for two years, and a 100,000 baht fine for allegations he made in a five-part article published by Manager newspaper and its website in May 2006.
Pramote Nakhonthap
The article accused Thaksin and his then-Thai Rak Thai Party of planning to destroy the Thai bureaucratic system, introduce single-party politics and overthrow the highest institution.
Thaksin sued him for defamation. The court found in Thaksin's favour, ruling that Pramote had produced no proof to support his allegation.
Pramote went to the Appeals Court, which upheld the lower court's judgement.
The prison sentence was suspended because Pramote had contributed to the nation through his work as an academic, as an advocate of democracy and as a columnist. The court found he had acted wrongly because he wanted to protect the highest institution.
The so-called "Finland Declaration" was a plan allegedly drawn up at a meeting in Finland in 1999 involving Thaksin and others to change the way Thailand is governed. The report was never substantiated.