Iain Duncan Smith has called on the prime minister to boycott a speech by Robert Mugabe at next week's earth summit in South Africa.

The Tory leader said it would be wrong for Tony Blair to "share a platform" with the Zimbabwean president.

In a letter to the prime minister, Duncan Smith said Blair must avoid giving the impression that he is working alongside Mugabe.

"I believe you should boycott the Mugabe address," he said.

"You could not possibly share a platform with someone who seeks to humiliate our country and place British citizens at great risk."

Number 10 has dismissed the suggestion that Blair will share a platform with Mugabe. A spokesman said: "There is certainly no question that they could possibly meet."

The prime minister has also been accused of neglecting the summit after it emerged he will spend less than a day in South Africa.

His speech is expected to last less than five minutes.