Zimbabwe's election chief has resigned citing ill health, state media reports.Simpson Mutambanengwe resignation comes as Zimbabwe gears up for a crucial constitutional referendum and elections later this year.Prime Minister Morgan Tsvangirai had been pushing for key reforms to guarantee free elections.The poll will herald the end of the shaky coalition he formed with President Robert Mugabe after the 2008 election, which was marred by violence.Mr Tsvangirai - the leader of the Movement for Democratic Change (MDC) - boycotted a run-off vote in 2008 after claiming he had been robbed of victory during the first round.Mr Tsvangirai and Mr Mugabe are expected to run against each other in the election.Mr Mugabe's Zanu-PF party was also accused of unleashing violence against MDC supporters, an allegation it denied.Justice Minister Chinamasa said that Mr Mugabe had accepted the resignation of Mr Mutambanengwe, a retired judge who took the post after the controversial 2008 poll, the state-owned Herald newspaper reports."The resignation is on the grounds of health," Mr Chinamasa is quoted as sayingHis replacement would be chosen after discussions involving the coalition partners, Mr Chinamasa added.Last month, New York-based campaign group Human Rights Watch called for reforms to the electoral body, judiciary and media, alleging they were loyal to Zanu-PF.Following talks later in the month, Mr Mugabe and Mr Tsvangirai said they had reached a deal over a new constitution, removing a key obstacle to elections.They said the constitution would be put to a referendum.Once approved by voters, elections are to be held.