An opposition political party that attempted to nominate a member of the royal family as prime minister in Thailand’s upcoming election has been dissolved with immediate effect and its executives given a 10-year ban from politics.

In a ruling announced on Thursday afternoon, Thailand’s constitutional court declared that Thai Raksa Chart, a political party formed only in October last year, had violated the constitution by putting forward Princess Ubolratana Rajakanya Sirivadhana Barnavadi, the older sister of the king, to be its chosen candidate for prime minister.

The court’s statement described Ulbolratana’s nomination as a “devious scheme” designed to give Thai Raksa Chart a political advantage which was both “hostile to the constitutional monarchy” and in breach of the Political Parties Act 2017. The dissolution of the party comes less than three weeks before the 24 March election.

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The decision is a blow to supporters of pro-democracy parties, and those loyal to exiled former prime minister, Thaksin Shinawatra, who were hoping that Thai Raksa Chart would help prevent a pro-military coalition taking control of the parliament. In an electoral first for Thailand, the military has formed a political party to run in the election with the intention of holding on to power even after the return of democracy.

Standing outside the court, the party leader, Preechapol Pongpanich, said: “We all had good intentions for the country.”

This month’s election will be the first since the military took over in a coup in 2014, and is Thailand’s first official poll since 2011, after a turbulent 2014 election was ruled void.

The announcement of the princess’s candidacy on 8 February shocked Thailand. The constitution stipulates that no member of the Thai royal family can hold political office, but Ubolratana renounced her titles in 1972 when she married an American, meaning that she legally holds no more privileges than an ordinary citizen.

However, 14 hours after Thai Raksa’s announcement, the king released a damning royal decree, stating that Ubolratana did not have his approval to run. He declared it “improper and highly inappropriate” for her to step into politics, forcing Thai Raksa to issue an apology and for Ubolratana to be disqualified from running by the election commission.

The party was reported to the election commission, which referred the case to the constitutional court with the recommendation that Thai Raksa Chart be dissolved.

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Despite efforts by the Thai Raksa executive to fight their case to the court the decision to dissolve the party did not come as a surprise to most in Thailand. The king holds absolute power in Thailand and his vehement opposition to Ubolratana’s decision to run put Thai Raksa Chart in a precarious position.

The dissolution was also seen as an opportunity for the military government to dent support for parties allied to Thaksin. Even from exile, Thaksin has huge loyalty across Thailand and is seen as a de facto head of Pheu Thai, the pro-democracy party he founded.

It was Thaksin who masterminded the creation of Thai Raksa Chart party last October as a way to get around the new constitution, which was drawn up by the junta to favour the continuation of military rule, and it is believed that Thaksin and Ubolratana, who are friends, came up with the idea of her entering politics together. The extent to which the princess thought she had the approval of the king to run for prime minister is still a source of debate.

Thai Raksa Chart was running in over 200 constituencies, and had expected to take between 20 to 40 seats to boost the pro-democracy opposition, which would include Pheu Thai and likely smaller parties such as Future Forward. The dissolution of Thai Raksa leaves around 100 constituencies without a pro-Thaksin party candidate.

Katherine Gerson, Amnesty International’s Thailand campaigner, said: “This decision highlights the Thai authorities’ abuse of judicial powers to restrict the peaceful association and expression of the political opposition. This far-reaching measure raises strong concerns about the human rights to freedom of association and expression in the period leading to the elections.”