Thai Prime Minister Paetongtarn Shinawatra faced mounting calls to resign on Thursday after a leaked phone call she had with former Cambodian leader Hun Sen provoked widespread anger and a key coalition partner to quit.

The coalition government led by Paetongtarn's Pheu Thai party is on the brink of collapse, throwing the country into a fresh round of political instability as it seeks to boost its spluttering economy and avoid US President Donald Trump's swingeing trade tariffs.

The conservative Bhumjaithai party, Pheu Thai's biggest partner, pulled out on Wednesday saying Paetongtarn's conduct in the leaked call had wounded the country and the army's dignity.

Losing Bhumjaithai's 69 members of parliament leaves her with barely enough votes to scrape a majority in parliament, and a snap election looks a clear possibility barely two years after the last one in May 2023.

Two coalition parties, the United Thai Nation and Democrat Party, will hold urgent meetings to discuss the situation later on Thursday.

Losing either would likely mean the end of Paetongtarn's government and either an election or a bid by other parties to stitch together a new coalition.

The Palang Pracharath party, which led the government up to 2023 and is headed by General Prawit Wongsuwan – who supported a coup against Paetongtarn's aunt Yingluck – called on the premier to resign.

In a statement, the party said the leaked recording showed Paetongtarn was weak and inexperienced, and incapable of managing the country's security.

"This already has proved that Thailand has a leader who will lead the country to a bad situation and weakness," it said.

Another opposition party, Thai Sang Thai, also called on Paethongtarn to step down, saying her conversation with Hun Sen had damaged the kingdom's sovereignty and the army.

In the leaked phone call, Paetongtarn is heard discussing an ongoing border dispute with Hun Sen – who stepped down as Cambodian prime minister in 2023 after four decades but still wields considerable influence.

She addresses the veteran leader as "uncle" and refers to the Thai army commander in the country's northeast as her opponent, a remark that sparked fierce criticism on social media, particularly on Pheu Thai page and Royal Thai Army page.

Thailand's armed forces have a long played a powerful role in the kingdom's politics, and politicians are usually careful not to antagonise them.

The kingdom has had a dozen coups since the end of absolute monarchy in 1932, and the current crisis has inevitably triggered rumours that another may be in the offing.

If she is ousted she would be the third member of her family, after her aunt Yingluck and father Thaksin Shinawatra, to be kicked out of office.

The current Pheu Thai-led government has already lost one prime minister, former businessman Srettha Thavisin, who was kicked out by a court order last year, bringing Paetongtarn to office. (AFP)