Maseru - A fledgling party formed this week by a former heavyweight in Lesotho's cabinet became the third largest opposition group in parliament on Friday after 17 ruling party lawmakers switched sides to join him.

Science and Communications Minister Tom Thabane, who was regarded as number three in Prime Minister Pakalitha Mosisili's government, resigned on Monday and formed a new party after saying the administration of the tiny southern African kingdom had lost its way.

Thabane on Friday crossed the floor with 17 parliamentarians from the ruling Lesotho Congress for Democracy (LCD) to sit on the opposition benches.

"I can assure you that as time goes on some members of parliament from the ruling party will cross the floor and join my party," a confident and smiling Thabane said, referring to his newly-formed All Basotho Convention (ABC).

"We need to make people understand what my party stands for and that is only when a large number of MPs will cross the floor. That is going to happen very soon," he said.

With the floor-crossing, the opposition's strength in parliament has risen from 41 to 59 against the ruling party's 61.

Lehlohonolo Ts'ehlana, a member of the ABC's executive committee, meanwhile issued a veiled threat to the government.

"We have to form a coalition with other opposition parties. We also need two more MPs to cross over to us so that we can propose a motion of no-confidence," he said.

"We are going to do that and those two MPs that we need will cross the floor very soon," he said.

Thabane was seen as the heir apparent to Mosisili while serving as foreign minister but he was sidelined in a recent government reshuffle.

He denied his shock resignation was due to thwarted ambition within the LCD. His replacement at the foreign ministry, Monyane Moleleki, is now regarded as the most likely successor to Mosisili.

The 61-year-old Mosisili, who has been prime minister since 1998, had been expected to stand down before the next general election but he later decided to stand for re-election as LCD chief, winning comfortably in January.

The general election is expected to take place in around six months' time in tiny Lesotho, which is surrounded by South Africa, although no date has yet been set. - Sapa-AFP