Botswana's main opposition party is challenging in court the results of 16 parliamentary constituencies won by the ruling Botswana Democratic Party (BDP) in last month's election, party spokesperson Moeti Mohwasa told Reuters.

The BDP was re-elected with 38 seats, but it faced a strong opposition challenge after former president Ian Khama fell out with President Mokgweetsi Masisi, his hand-picked successor, and backed The Umbrella for Democratic Change (UDC) party.

Should the High Court rule in the UDC's favour and strip the BDP of 16 seats, the ruling party would be left with 22 seats and fall short of a majority in the 57-seat parliament.

"We have filed petitions in 16 parliamentary seats and five council positions over vote fraud. The challenge is mostly on the grounds of suspicion of double-voting by our rivals," Mohwasa said.

The UDC, a coalition of three parties, won 15 seats in the October election.

It was hoping to perform better than in 2014, when it got 18 seats, but an overwhelming win by the BDP in the southern part of the country, traditionally an opposition stronghold, ensured Masisi's party retained power.

Registrar of the High Court Michael Motlhabi said in a statement that election petitions were urgent and had to be tried and determined within 90 days. The UDC's petition would be heard by a panel of judges, Motlhabi said.

The BDP has ruled Botswana since independence from Britain in 1966, and the country has enjoyed stability and prosperity thanks to its plentiful diamond resources.

At his swearing-in this month, Masisi promised to drive a transformation of Botswana's economy, creating more jobs and countering excessive dependence on diamonds.