Gordon Brown declared yesterday that the world must stop Robert Mugabe from stealing the Zimbabwean election and raised the prospect of a run-off contest supervised by United Nations monitors.

Challenging the Zimbabwean leader and abandoning nearly a decade of British “soft” diplomacy, the Prime Minister told a UN summit: “No one thinks having seen the results in polling stations that President Mugabe has won this election. A stolen election would not be a democratic election at all.”

Mr Brown was speaking at a Security Council meeting in New York chaired by Thabo Mbeki, the South African President, who did not even mention Zimbabwe in his speech. Mr Mbeki has continued to advocate his policy of “quiet diplomacy” towards Zimbabwe, which until yesterday was supported by Britain.

In what was regarded as a diplomatic snub, the South African leader cancelled a scheduled meeting with Mr Brown and instead chatted to him for only a few minutes in a UN lounge. Mr Mbeki later made a veiled attack on Mr Brown by criticising “loud diplomacy”. “It’s not diplomacy; it cannot be,” Mr Mbeki said.

Mr Brown and Zalmay Khalilzad, the US Ambassador to the UN, joined forces to embrace a proposal by Ban Ki Moon, the UN Secretary-General, to send international observers to monitor a possible run-off between Mr Mugabe and Morgan Tsvangirai, the opposition leader.

“The credibility of the democratic process in Africa could be at stake here,” the UN chief told the summit. “If there is a second round of elections, they must be conducted in a fair and transparent manner, with international observers.”

France joined the growing international chorus. Rama Yade, its Human Rights Minister, called for the Zimbabwean Government to release the election results. “The people of Zimbabwe need to know the truth,” she said.

But China, which maintains close relations with Mr Mugabe as part of its quest for natural resources in Africa, made no mention of Zimbabwe in its speech. Chinese diplomats say the election stand-off is an internal matter not appropriate for Security Council involvement.

A UN-monitored run-off could offer a compromise to end the impasse, but international monitors would need to be invited by the host country. Mr Mugabe could avoid defeat in the March 29 poll, while Mr Tsvangirai might be reassured that the result of the rematch would not be rigged.

Britain is still calling for the results of the election to be published, while emphasising that from all the available evidence it appears that Mr Mugabe lost.

British officials believe the issue must now be addressed by the Southern Africa Development Community (SADC), which is due to meet again this weekend. Mr Mbeki announced that SADC representatives would travel to see the Zimbabwe Election Commission to search for a way to verify the election results.

Should the SADC conclude that the most sensible way forward is a run-off, Britain wants UN oversight, including election monitors.

Mr Brown discussed the idea with Mr Ban at breakfast yesterday, a day after the UN chief had spoken to President Bush. “Everybody is angry they have not seen the election results announced,” Mr Brown said. “The General-Secretary has announced UN help and if there were to be a second round, they would send observers . . . I am pleased the UN Secretary-General is dealing with the situation.”

The United States proposed that the UN and African Union send a joint delegation to press Zimbabwe’s election commission to publish the results of the March 29 poll.

It was unclear last night how badly damaged Britain’s relations were with the South African leader, a close ally of Tony Blair’s and new Labour. Mr Mbeki, who has called the situation in Zimbabwe a “normal electoral process”, had insisted it was not on the Security Council agenda.

The South African leader earlier cancelled a scheduled sit-down meeting with Mr Brown in a private room because of a “diary clash”. The meeting was replaced by what a British official called a five to ten-minute “brush-past” in a diplomatic lounge backstage before the two walked into the Security Council together.

However, at a press conference a few hours after their chilly encounter, Mr Mbeki struck a conciliatory tone, acknowledging that “there are many wrong things with the politics of Zimbabwe”, and urging its Government to cut down on the violence.

“If that second round takes place, it needs to be handled in the same way as the first round was handled, where there was really no violence, everyone was free to campaign everywhere in the country,” Mr Mbeki said.

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