THAILAND'S most popular politician was banned from politics for 10 years, and his party dissolved on Wednesday following his attempt to reform archaic royal defamation laws.

The Constitutional Court in Bangkok voted unanimously to dissolve the Move Forward Party (MFP) and ban its executive board, which includes its former leader Pita Limjaroenrat, for 10 years, Judge Punya Udchachon said.

Pita, 43, led the reformist MFP to a shock first place in a general election last year after striking a chord with young and urban voters with his pledge to reform Thailand's strict royal defamation law.

"Let's be sad today for one day, but tomorrow we will move on, and let's release the frustration through the next ballot we will cast in the next election," Pita told a news conference on Wednesday evening.

His bid to become prime minister was blocked by conservative forces in the Senate.

A coalition of army-linked parties took office instead under Prime Minister Srettha Thavisin.

The European Union, the United States, the United Nations and human rights groups blasted the court's decision, which the EU said harmed democratic openness in Thailand.

"No democratic system can function without a plurality of parties and candidates," an EU spokesman said in a statement.

US State Department spokesman Matthew Miller said the ruling ran "counter to the aspirations of the Thai people for a strong, democratic future," according to a statement.

"It's a setback to pluralism and democracy in Thailand and fundamental freedoms of association and expression," Farhan Haq, deputy spokesman for the UN secretary-general, told reporters.

Pita's political career was already shaken in March, when Thailand's election commission asked the top court to dissolve the MFP.

That followed a ruling that the party's pledge to reform the lese-majeste law amounted to an attempt to overthrow the constitutional monarchy.

Lese-majeste charges are extremely serious in Thailand, where King Maha Vajiralongkorn enjoys a quasi-divine status that places him above politics.

Dozens of supporters dressed in the party's trademark orange gathered in front of MFP headquarters in Bangkok.

Pita appeared in parliament in high spirits earlier on Wednesday, telling lawmakers he had faith in the kingdom's legal process.

He warned against the weaponization of Thailand's judicial system, saying in an interview with AFP before the ruling that 33 parties had been dissolved over the past two decades, including "four major ones that were popularly elected."

"We should not normalize this behavior or accept the use of a politicized court as a weapon to destroy political parties," he said.

The MFP has 148 seats in Thailand's 500-seat parliament, and Pita said its executive would form a new vehicle if the party was dissolved.

The MFP later said it would be relaunched on Friday.

Pita first appeared on the political scene in 2018 as part of the progressive Future Forward Party (FFP), which was dissolved in 2020, sparking mass youth-led demonstrations that shook Bangkok for months.

Tens of thousands took to the streets at the height of the protests, many making unprecedented public criticism of the royal family as well as demands for transparency and reform.

More than 270 people were charged with lese-majeste after those protests, including two elected MPs.

New York-based Human Rights Watch (HRW) says the lese-majeste law has been routinely used to silence political dissent.

Thailand, Southeast Asia's second-largest economy, is known for its chronic instability, with a dozen coups since the end of absolute monarchy in 1932.