Thomas Thabane, 80, who faces possible prosecution over the killing of his ex-wife, told AFP in a telephone interview on Tuesday that he is retiring because of old age.
Thabane and his then wife Lipolelo Thabane, 58, were going through a bitter divorce when she was shot and killed outside her home two days before her husband’s 2017 inauguration.
Police have since found Thabane’s mobile number in communications records from the crime scene — prompting rivals within his party to demand his immediate resignation.
He briefly appeared in court in February accused of acting in “common purpose” in the killing, but he was not formally charged and the issue is yet to be decided.
Thabane, whose five-year term was due to run until 2022, said he felt ready to leave and was tying up loose ends to ensure his retirement was “as smooth as possible”.
His coalition government was dissolved this week, with parliament due to meet on May 22 to form a new government and name a new prime minister.
Thabane’s All Basotho Convention (ABC) has nominated Finance Minister Moeketsi Majoro as his successor.
Thabane must inform King Letsie III — the supreme traditional leader of the southern African kingdom — of his intention to step down.
“I will have to go and inform him that I am leaving,” Thabane said. “Normally he would say ‘fine’.”
The prime minister said he expected to submit the letter on Wednesday but did not give a precise date for stepping down.
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