Margaret Beckett has dismissed suggestions that Tony Blair should stage a walkout from next week's earth summit in protest at Robert Mugabe's regime.

Her comments came as Iain Duncan Smith called on the prime minister to boycott a speech by the Zimbabwean president at the gathering of world leaders in South Africa.

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

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."

But Beckett said it would be wrong to boycott the talks and stressed that the government had made plain its opposition to Mugabe's regime.

"We are certainly determined to make sure that the important issue of the summit and its potential to do good isn't hijacked by issues, by concerns such as those about Zimbabwe," he said.

"The Zimbabwean government is under no illusion about the British attitude to the policies that they have been pursuing. Indeed, some of the rudest things that have ever been said about Tony Blair have been said by Robert Mugabe because of the British government's opposition."

Duncan Smith warned the prime minister that Mugabe is "systematically starving his own people, driving efficient farmers off highly productive land and forcing farm workers to live in squatter camps".

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