Twenty-one opposition MPs have been expelled from Zimbabwe's parliament after they broke away to form a new party.

Speaker Jacob Mudenda ordered the expulsion of the MPs, led by former Finance Minister Tendai Biti.

The group had contested the 2013 election under the banner of Morgan Tsvangirai's Movement for Democratic Change (MDC) party.

They later split to form MDC Renewal after rejecting his leadership.

The expulsions pave the way for by-elections in the constituencies of the 14 MPs who were directly elected, reports the BBC's Brian Hungwe from the capital, Harare.

The MDC will be able to nominate replacements for the other seven seats as they were allocated according to the proportion of votes cast in the last election, he says.

The MDC served as a junior partner in a coalition government from 2009 until elections in 2013 in which President Robert Mugabe's Zanu-PF party was victorious.

MDC Renewal blamed Mr Tsvangirai, who served as prime minister in the coalition government, for its defeat and demanded his resignation.

He refused, and accused the breakaway faction of being manipulated by Mr Mugabe.

Our correspondent says Mr Biti was not in the chamber when he and his MPs were expelled.

The affected members were escorted out of parliament looking dejected after the speaker's announcement, he says.

It was the second split in the MDC. In 2005, Mr Biti's predecessor as MDC secretary-general, Welshman Ncube, broke away to launch his own faction.

Mr Mugabe, 91, and Zanu-PF have been in power in Zimbabwe since independence in 1980.