Archbishop of Canterbury Rowan Williams says he is "amazed" the policy of sending failed asylum seekers back to troubled Zimbabwe has not been halted.
Dr Williams wants African nations to put pressure on Mugabe
Charles Clarke has rejected calls for a suspension of all removals - despite concerns about human rights abuses by President Robert Mugabe's regime.
The home secretary says each case would be treated individually on its merits.
But Dr Williams told BBC Radio 4's Today programme: "I think it's deeply immoral to send people back there."
He was speaking as a report in The Times claimed immigration officers had been ordered to halt deportations to Zimbabwe - despite Tony Blair claiming on Monday that there would be no official suspension of forced removals.
Dozens of Zimbabweans in detention centres across the UK have gone on hunger strike.
Conservative shadow home secretary David Davis called UK Zimbabwe policy a "miserable failure".
Dr Williams told Today the asylum system "isn't working" and called for the government's policy on Zimbabwe to be reviewed.
He also urged Zimbabwe's neighbours - particularly South Africa - to "rally round" and put pressure on the Mugabe regime.
The head of the Church of England said he had visited a number of detention centres where failed asylum seekers were being held pending deportation.
"You are often dealing with people who have been here for many years and have roots in the country and are suddenly, without warning, taken into the system," he said.
"I think there is a lot in the working of it which is deeply unsatisfactory at the moment, which feels inhuman to the people involved."
However, the home secretary has argued that not all Zimbabweans who claim asylum in the UK genuinely face persecution.
He said that of those who had been returned there had been "no substantiated reports of mistreatment".
On Monday Mr Blair argued that halting all deportations could send a signal around the world "that Britain is open for business" even for failed asylum seekers.
Downing Street reiterated that message on Tuesday, insisting there would be "no change in policy" over the deportation of failed asylum seekers to Zimbabwe.
Crimes against humanity?
The prime minister's spokesman said there had been clear abuse of the asylum system with people falsely claiming to come from Zimbabwe.
But Mr Davis said Robert Mugabe's regime was guilty of "crimes against humanity on a massive scale".
Shadow foreign secretary Liam Fox said he found it "bizarre" that the government had effectively held an amnesty for 250,000 economic migrants but was endeavouring to send back people who would be persecuted.
A ban on deportations to Zimbabwe, which had been in force for two years, was lifted last November.
During the first three months of this year 95 Zimbabweans were sent home.
Recent moves in Zimbabwe to demolish illegal buildings - which the UN says has left 275,000 people homeless - have drawn objections from the Foreign Office.