Thai Prime Minister Paetongtarn Shinawatra's coalition government is on the brink of collapse after a phone call between her and former Cambodian leader Hun Sen about a festering border dispute was leaked.

The leak provoked public anger and prompted a key coalition partner of the 38-year-old Paetongtarn's Peu Thai party to quit.

In the call, she addressed Hun Sen, a family friend and senior politician in the South East Asian region, as "uncle" and appeared to dismiss a Thai military commander.

"I would like to apologise for the leaked audio of my conversation with a Cambodian leader which has caused public resentment," Paetongtarn said on Thursday, as the pressure on her intensified.

Bhumjaithai, the second-largest party in Paetongtarn'sruling coalition, quit the alliance on Wednesday, dealing a major blow toher party's position in parliament.

Her coalition now holds a slim majority - which will be lost if more of its partners decide to leave. Two other coalition partners will meet later on Thursday to discuss the situation.

Critics took issue with the PM's apparent deference to Hun Sen when she addressed him as "uncle" and promised to "take care" of his needs.

They also accused her of undermining the country's politically influential army. She had told Hun Sen in the call that a Thai military commander handling the most recent flare-up of border tensions "just wanted to look cool and said things that are not useful".

The Shinawatras' friendship with Cambodia's Hun family goes back decades. Hun Sen and Paetongtarn's father, former Thai Prime Minister Thaksin Shinawatra, consider each other "godbrothers".

The Thai leader defended the call as a "negotiation technique" but opposition figures have called on her to resign.

Hun Sen said he had shared the audio clip with 80 politicians and one of them leaked it. He later shared the entire 17-minute recording on his Facebook page.

Paetongtarn is just 10 months into the job. She took over as prime minister last August, after her predecessor Sretta Thavisin was removed by the country's constitutional court for violating a rule on cabinet appointments.

She is the daughter of Thaksin Shinawatra, the deposed former PM who returned to Thailand last August after 15 years in exile. She is also the youngest prime minister in Thailand's history, and only the second woman - the first was her aunt Yingluck Shinawatra.

'Deeply disappointed' with leak

In a letter to the Cambodian ambassador, Thailand's foreign ministry said it was "deeply disappointed" over the leak of a "private telephone conversation".

"Trust and respect between the two leaders are fundamental to good neighbourliness and conduct among states," the letter read.

It also said the leak "will severely affect ongoing efforts for both sides to resolve the problem in good faith".

Paetongtarn has said that she would no longer engage in private talks with the Cambodian leader.

Tensions at the border ramped up in May after a Cambodian soldier was killed in a clash. This plunged bilateral ties to their lowest in more than a decade.

Cambodia has banned imports from Thailand, ranging from fruit and vegetables to electricity and internet. It also banned Thai dramas from TV and cinemas as a result of the border dispute.

Both countries have imposed border restrictions on each other.

The dispute between them dates back to more than a century, when the borders were drawn after the French occupation of Cambodia.