HARARE (Reuters) - Zimbabwe’s High Court on Wednesday declared Nelson Chamisa an illegitimate leader of the main opposition MDC party and ordered the movement to convene an extraordinary congress to choose a new leader after a month.

The Movement for Democratic Change (MDC) immediately said it would appeal the ruling and that it would press on with a planned leadership congress this month where it is expected to elect Chamisa as a permanent successor to founding leader Morgan Tsvangirai, who died in February 2018.

Chamisa, 41, rose to become the MDC interim leader last year amid opposition from internal rivals, which split the party into two factions. He lost a presidential election to the ruling ZANU-PF’s Emmerson Mnangagwa, but says that poll was rigged.

High Court Judge Edith Mushore ruled that the process that took Chamisa to the helm of the MDC was unconstitutional and therefore null and void. The ruling followed a legal challenge from a party member to Chamisa’s leadership.

But the MDC said the ruling was part of a wider plot by Mnangagwa’s ruling party to destabilize the main opposition.

“We fundamentally disagree with the judgment. We reiterate that Emmerson Mnangagwa is illegitimate and no amount of diversionary tactics will change this fact,” MDC spokesman Jacob Mafume told reporters.

Chamisa has already been nominated for the presidency of the MDC by all the party’s provincial assemblies and will be endorsed during a congress from May 24-26 in Gweru town, central Zimbabwe.

In February this year, Chamisa and his MDC snubbed talks with Mnangagwa meant to try to resolve a political and economic crisis, saying any dialogue with the president must be brokered by an independent outside mediator.