By Robert Pigott
BBC Religious Affairs correspondent
The Archbishops explain their concerns about Zimbabwe
The Archbishop of Canterbury, Dr Rowan Williams, has called on Anglicans to "pray, fast and give" to highlight Zimbabwe's slide toward starvation.
Dr Williams and the Archbishop of York, John Sentamu, have chosen the growing crisis in Zimbabwe for their first joint appeal for funds.
They said people should give now rather than wait for a political solution.
Dr Sentamu said if people did not give, disease and starvation would mean "more and more graves".
In a joint interview with Dr Williams for BBC News, Dr Sentamu said he will spend Wednesday fasting in St Helen's Church in York, as well as leading hourly prayers for Zimbabwe.
I think the humanitarian crisis in Zimbabwe is now at an appalling level
Fasting is a traditional act of penitence and reflection that Christians use to mark Ash Wednesday and the beginning of Lent.
Dr Williams called on people around the world, especially Anglicans, to fast, pray and give for Zimbabwe.
Otherwise, he said, disaster was inevitable.
Archbishop Sentamu said that his simple day of prayer and fasting would provide a sharp contrast with the elaborate schedule of the president of Zimbabwe, Robert Mugabe.
It has been reported that Mr Mugabe will be spending at least part of Wednesday at the Rainbow Towers Hotel in Harare - half way through a week of sumptuous celebrations for his 85th birthday.
He will be attending a dinner, costing a reported £70 ($100) a ticket, and will be serenaded by a variety of musical bands, both home-grown and recruited from overseas.
Thousands of Zimbabweans, many of them children, have contracted cholera
Dr Williams said that beyond the gilded world occupied by the President and his entourage, Zimbabweans faced desperate conditions.
"I think the humanitarian crisis in Zimbabwe is now at an appalling level," he said.
"It's estimated that perhaps half the population is now under threat of starvation; and the deaths from cholera have been climbing in just the last couple of weeks from 3,000 towards 4,000.
"Everyone knows about the rate of inflation, but I think the main thing is the sheer level at which people are at risk of starvation."
"People will die. They'll die quickly, unpleasantly, and children and young people will bear the brunt of it."
Archbishop Sentamu said many of those deaths would come from disease.
"The spread of cholera, I'm afraid, will just increase because there isn't clean water. So in the end, if people don't heed this particular appeal, there'll be more and more graves".
Archbishop Sentamu has been scathing about Robert Mugabe, and publicly cut up his clerical collar on BBC television in 2007, promising to go without one until the President left office.
Despite Zimbabwe's desperate plight, the UN's World Food Programme recently reported that donor countries had actually reduced the amount they were giving.
The UN said donors were apparently waiting to see what would result from the power-sharing deal between President Mugabe and Morgan Tsvangirai before committing themselves to further funding.
Dr Sentamu - still without a clergy collar - told people they should give money now, rather than waiting in the expectation of a political solution in Zimbabwe.
"There is this seeming political solution by power sharing, but the truth is, as long as the Home Office Department is not controlled other than by Mugabe and his friends, just forget it! Because security won't return and people won't feel safe," he said.
Anglicans' relations with President Mugabe are already extremely tense.
After the Church criticised him last year, priests said that twenty Anglican churches were targeted by the police.
In one case, a priest told the BBC how officers armed with sticks interrupted a service and ordered the congregation of 300 to leave.
A number of women were struck as they knelt in front of the altar in the act of taking the bread and wine of communion.
Dr Williams praised Anglicans in Zimbabwe for the "courage and imagination" they had shown in facing danger, and insisted that despite the rift with the government, the Church was well placed to deliver aid.
He also called for political sanctions.
"Sanctions that cut against the people of Zimbabwe are gong to be massively counter-productive.
"Sanctions against the political legitimacy and acceptability of the ruling elite in Zimbabwe are, I think, necessary.
"I hope they're effective, but they're not the whole story".
President Mugabe often portrays criticism from Britain as a throwback to colonialism.
But it's hard to pin that label on Archbishop Sentamu.
Dr Sentamu's Ugandan ancestry, his senior position in the Anglican Church (which has many members in Zimbabwe), and his reputation for sustaining campaigns such as this, make him an effective critic of Robert Mugabe.
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