International Development Secretary Douglas Alexander said Mr Mugabe had shown a "callous contempt for human life" in his struggle to cling to power.

The move, announced last night in Harare, sparked fresh fears of an orchestrated campaign of intimidation ahead of the run-off later this month in the country's disputed presidential elections.

In a statement, Mr Alexander dismissed claims by the Zimbabweans that aid agencies had been involved in campaigning for the opposition as "offensive and absurd".

"For Robert Mugabe to use the threat of hunger as a political weapon shows a callous contempt for human life," he said.

"This indefensible decision, which seems to be part of a strategy ahead of the presidential run-off, demonstrates to the world the lengths to which Mugabe will go to cling on to power.

"For the sake of millions of the poorest and most vulnerable people in Zimbabwe, aid must be allowed to get through."

Opposition parties joined the condemnation of the regime's actions and called for the ban on the agencies to be lifted.

"This cruel and callous act will put thousands of lives at risk," said shadow international development secretary Andrew Mitchell.

"I call for the immediate lifting of these restrictions and unhampered access for humanitarian agencies to all of Zimbabwe," he said.