HARARE (Reuters) - Zimbabwe’s main opposition leader Morgan Tsvangirai on Saturday reunited with his former allies to agree a pre-election pact to challenge President Robert Mugabe’s near four-decade hold on power at the polls next year.

FILE PHOTO - Morgan Tsvangirai, leader of Zimbabwe's main opposition Movement For Democratic Change (MDC), addresses his supporters during a march against what they say is the mishandling of the economy by President Robert Mugabe's government in Gweru, August 13, 2016. REUTERS/Philimon Bulawayo

Mugabe, 93, has ruled the former British colony since independence in 1980.

Tsvangirai’s Movement for Democratic Change (MDC) has been the main threat to Mugabe since its formation in 1999 but has been weakened by splits, in 2005 and 2014, mainly over strategy.

On Saturday, Welshman Ncube, who led a break-away MDC faction in 2004 and Tendai Biti, who left the main opposition in 2014, signed a pact that would see them fielding parliamentary candidates in some constituencies under the MDC Alliance banner and would support Tsvangirai’s fourth bid for the presidency.

Ncube and Biti are founding MDC members who both held the position of secretary general and served as cabinet ministers in a unity government with Mugabe’s ZANU-PF until a crushing defeat in the 2013 election.

Two other smaller political parties led by a former army general and Mugabe ally Agrippa Mutambara and Jacob Ngarivhume are part of the election alliance that was signed in Highfield township in the capital Harare.

“We are saying today, this convergence is not just about political parties. It is to stop fragmentation. Mugabe has no excuse to rig if we are united,” Tsvangirai told supporters after the signing ceremony.

Biti added: “We owe it to the thousands of Zimbabweans to make sure that in our lifetime we can remove the beast called ZANU-PF.”

The opposition accuse Mugabe of rigging his way to victory in previous elections and say he has used political violence and the state security apparatus to maintain his hold on power. Mugabe denies the claims.

The president plans to contest next year’s vote, his last under the constitution, and will be 99 if he wins and finishes the five-year term.

An election pact signed in April between Tsvangirai and Joice Mujuru, who was Mugabe’s vice president for a decade until she was fired in 2014, was on the verge of collapse, mainly over leadership of the alliance, officials from both camps have said.