Thailand's Constitutional Court on Thursday ordered the dissolution of an opposition party for nominating the king's sister as its candidate for prime minister in a March 24 election.

Banning the Thai Raksa Chart party just over two weeks before the general election struck a blow to opposition parties' chances of defeating parties allied to the military junta that has ruled since a coup d'etat in 2014.

"The court has ordered that the party be dissolved," Judge Taweekiet Meenakanit said in the ruling, which also banned the party's executive board members from politics for 10 years.

Thai Raksa Chart is one of several parties loyal to ousted former Prime Minister Thaksin Shinawatra in an election that broadly pits his supporters against establishment parties, including one that has junta leader Prayuth Chan-ocha as its prime ministerial candidate.

Police had deployed more than 1,000 officers in and around the court and cordoned off the surrounding area ahead of the ruling.

Party leader Preechapol Pongpanich, who was visibly emotional, told reporters outside the court the party accepted the ruling.

"We all had good intentions for the country," he said.

Supporters were seen crying, saying they would vote instead for other opposition parties.

The Election Commission asked the court to dissolve the party after it nominated Princess Ubolratana Rajakanya Sirivadhana Barnavadi for prime minister, which the commission described as "antagonistic toward the constitutional monarchy".

The princess relinquished her royal titles in 1972 but is still officially treated as a senior member of the royal family, which is highly revered in Thailand.

King Maha Vajiralongkorn strongly opposed his sister's nomination, calling it "inappropriate" and unconstitutional and her candidacy was swiftly disqualified by the Election Commission.

While Thai Raksa Chart is not the largest pro-Thaksin party, dissolving it would weaken his supporters' strategy of fielding several separate parties in the hope of securing more seats for Thaksin loyalists.

'MOVING AHEAD'

New electoral rules put in place by the outgoing military government benefit smaller parties at the expense of the top vote-winning parties when 150 "party seats" are shared out in the 500-seat House of Representatives.

The other 350 seats are directly elected.

"Dissolving Thai Raksa Chart ... would lead to a kind of disenfranchisement because Thai Raksa Chart forms a large chunk of the pro-Thaksin party team," Thitinan Pongsudhirak, a political scientist at Chulalongkorn University, told Reuters before the ruling.

The princess, who broke with royal tradition by attempting to enter politics, is in Germany, according to her posts on Instagram.

She did not make a direct reference to the court ruling but said on Thursday: "Today, I want to continue moving ahead in working for Thailand."

Thaksin's opponents have sought to portray his movement as insufficiently loyal to the monarchy - which the self-exiled Thaksin vehemently denies - and Thai Raksa Chart's nomination of the king's sister was intended to refute that assertion.

But Thitinan said the banning of Thai Raksa Chart was unlikely to result in fewer votes for pro-Thaksin parties and more for pro-military ones. Voters who had been aiming to support it would simply switch to another pro-Thaksin party.

"The structure of Thailand's polarization with Thaksin and what he stood for as the fault line remains more or less the same," he said.