Lesotho's embattled Prime Minister Thomas Thabane announced his resignation on Monday, ending months of political uncertainty gripping the kingdom since he was accused of participating in his ex-wife's murder.

Thabane's political rivals have piled pressure on the prime minister to step down early over allegations that he had a hand in the brutal 2017 shooting of Lipolelo Thabane, whom he was divorcing.

His coalition was disbanded in parliament last week and he had been expected to resign by May 22, when a new government is due to be installed.

"I decided to personally come and inform you that I am stepping down as prime minister of Lesotho," Thabane told supporters in his Abia home constituency on the outskirts of the capital Maseru.

The 80-year-old did not state when he would formally quit but government spokesman Nthakeng Selinyane said the premier would officially "announce his resignation" in an address to the nation on Tuesday.

"It is rather difficult to part ways with something that you had been accustomed to and leave people behind, but we all have to leave at some point," Thabane added.

For the time being, he will remain leader of his ruling All Basotho Convention (ABC), he said.

Thabane, whose term is due to end in 2022, had in January set himself a target of leaving office by July 31 because of his advanced age.

But Lesotho has been plagued by political instability since the start of this year, when police said they found Thabane's mobile number in communications records from the scene of his estranged wife's murder.

The alleged evidence prompted rivals within and outside Thabane's party to demand his immediate resignation.

Prime Minister Thomas Thabane, seen here at a political rally in March, has announced his resignation after months of political uncertainty
Prime Minister Thomas Thabane, seen here at a political rally in March, has announced his resignation after months of political uncertainty AFP / MOLISE MOLISE

Lipolelo Thabane, 58, was shot and killed outside her home two days before her husband's inauguration in July.

The murder sent shock waves through Lesotho -- a tiny landlocked nation of 2.2 million with a history of political turmoil.

Thabane has denied involvement in the killing.

His current wife Maesaiah Thabane, 43, whom he married two months after Lipolelo's death, is considered a co-conspirator in the murder case.

She has already been charged with murder and is out on bail.

Thabane's government collapsed after his coalition partners pulled out over his repeated refusal to leave office earlier than stated.

The ABC and main opposition Democratic Congress party have nominated Finance Minister Moeketsi Majoro as Thabane's successor.

Lesotho's supreme traditional leader King Letsie III was advised by his council on Monday to appoint Majoro as prime minister, his senior private secretary Monehela Posholi said in a statement.

"Majoro will be sworn in as his successor tomorrow at 10 am (0800 GMT)," said ABC spokesman Montoeli Masoetsa.

The ABC's national executive committee is expected to hold a special conference in July to elect a new party leader.

While no premier has served out a full five-year term in Lesotho over the past decade, the octogenarian Thabane boasted in an interview with AFP last week that he has set an example to fellow African leaders who have a propensity to cling to power.

"I'm trying to set a precedent that leaders in Africa must volunteer to leave when they think it's time to leave or at the very worst they must leave when their term ends."