Thailand today freed from prison a Swiss man jailed for blackmail and attempted extortion in a case linked to a scandal over Malaysian state fund 1Malaysia Development Berhad (1MDB).

Xavier Justo was among 150,000 Thai and foreign inmates who were granted a royal amnesty this month by Thailand's King Maha Vajiralongkorn. He will be deported to Switzerland, his lawyer said.

Justo was jailed for three years in August 2015 after Thai police said he admitted to blackmail and attempted extortion of his former employer, Petrosaudi International, a Saudi Arabia-based oil and gas company and former joint-venture partner of 1MDB.

The Malaysian state fund 1MDB, once a pet project of Malaysian Prime Minister Najib Abdul Razak, who chaired its advisory board, is the subject of money-laundering investigations in at least six countries, including Switzerland, Singapore and the United States.

After his release, Justo, dressed in a white T-shirt, blue shorts and flip flops, was taken to an immigration detention centre in Bangkok in a police van accompanied by four police commandos.

"I'm happy to be sent home," Justo told Reuters as he went into the centre. "I hope to be back by tonight."

Justo's lawyer in Thailand, Worrasit Piriyawiboon, said his client would be deported to Switzerland.

"It's now up to immigration when they decide to deport him and up to his embassy," Worrasit told Reuters.

In August, Justo's sentence was cut to two years under a royal amnesty, prompting the Swiss authorities to request his transfer to serve the remaining term back home.

Thailand declined the extradition request, which Swiss authorities said in September they regretted.

Worrasit said it was customary for Thailand to deport people to their country when they are released, and Justo could do what he wanted when he got home.

"This is not an extradition. This is a deportation," he said. "Once he's in Switzerland, he's a free man."

Reuters was not able to reach the Swiss Embassy in Bangkok for comment.

Thai police said Justo leaked documents from Petrosaudi relating to its joint venture with 1MDB that were published by Malaysian and international media groups last year and formed the basis of the money-laundering investigation.

He was arrested on a Thai resort island in 2015 following allegations by Petrosaudi that he had demanded payment in exchange for not disclosing confidential information, Thai police said at the time.

Najib has denied any wrongdoing in connection with 1MDB and said Malaysia would cooperate with the investigations.

- Reuters