The Thai prime minister, Prayuth Chan-ocha, has said he will retire from politics, more than nine years after the former army general seized power in a military coup.

In May, his party suffered a humiliating defeat, coming fifth, well behind an upstart opposition party that promised to remove the military from politics. He will remain as caretaker prime minister until a new government is formed.

Prayuth, 69, first took power in a coup in 2014, presiding over the arrest of activists and political opponents, and censoring the media. He was later appointed prime minister by parliament in 2019.

The staunch royalist and conservative said in his resignation statement that he had worked hard to “protect the nation, religion, monarchy for the benefit of the beloved people.

“I have tried to strengthen the country in all areas for stability and peace and overcame many obstacles domestically and internationally.”

In office he has faced a number of no-confidence votes, as well as mass protests in 2020 that called for his removal from office, the scrapping of a military-backed constitution passed under his rule, and reforms to the monarchy.

He responded by threatening to use “all laws, all articles” against pro-democracy protesters.

While Prayuth may be stepping back, the undemocratic structures that were introduced under his rule remain in place, said Pavin Chachavalpongpun, an associate professor at the Centre for Southeast Asian Studies, Kyoto University. These include the military-appointed senate whose members will vote alongside the lower house of elected lawmakers to appoint a future prime minister on Thursday.

“This is not going to be the last man standing in the military,” Pavin said of Prayuth, referring to the enduring power of the institution.

The Move Forward opposition party, which campaigned with the motto “If you have uncles, you don’t have us” – a reference to its refusal to enter coalition deals with any of the army generals involved in the 2014 coup – won the most seats in May’s election.

However, it is unclear if its leader, Pita Limjaroenrat, will secure support from enough military-appointed senators to take power.

Some have said they will not support Pita because they disapprove of his party’s pledge to reform the country’s strict lese majesty law, under which criticism of the monarchy can lead to up to 15 years in prison. More than 250 people, including children, have faced charges under the law since the 2020 pro-democracy protest movement.