Military intervention in Zimbabwe would be justified to stop the violence there deteriorating into mass slaughter, Paddy Ashdown told The Times last night.

Lord Ashdown of Norton-sub-Hamdon said: “The situation in Zimbabwe could deteriorate to a point where genocide could be a possible outcome - something that looks like [another] Rwanda.”

In that case, international military action, with Britain playing a “delicate role”, would have to be considered, said the former European Union High Representative for Bosnia and Herzegovina. Britain and the United States became the first countries to refuse to recognise Robert Mugabe as President of Zimbabwe yesterday after Morgan Tsvangirai, leader of the opposition Movement for Democratic Change (MDC), sought refuge at the Dutch Embassy.

The Dutch Government said that Mr Tsvangirai had been granted sanctuary soon after announcing he was withdrawing from the run-off presidential election on Friday because of mounting violence that has killed more than 85 supporters, left hundreds wounded and tens of thousands displaced. Yesterday Zimbabwean police raided the MDC’s headquarters, seizing 60 of its supporters.

The Times has learnt that the Ministry of Defence already has two contingency plans, one involving the deployment of troops into Zimbabwe.

Lord Ashdown and Lord Carrington, the former Foreign Secretary, who led the negotiations that brought to an end white rule in Rhodesia, said that the African Union was the ideal organisation to deal with President Mugabe.

Britain pressed for fresh sanctions against Zimbabwe yesterday and asked the United Nations to recognise the opposition’s claim to power, after Condoleezza Rice, the Secretary of State, said that Mr Mugabe’s Government could not be considered legitimate without a fair run-off.

Gordon Brown said that the status quo could not be allowed to stand. “We will not recognise the fraudulent election rigging and violence and intimidation of a criminal and discredited cabal,” he said.

The UN Security Council last night agreed to condemn the Zimbabwe government after a UN official reported a “staggering degree of violence”. “There is ample evidence that such violence has been perpetrated by a combination of state agencies - army, police and intelligence - war veterans and youth militias,” Lynn Pascoe, the UN’s political supremo, told the council.

At Britain’s urging, the 15-nation council was poised to declare that the “campaign of violence” had made it “impossible” to hold a run-off election on schedule on Friday.

But South Africa blocked a British bid to get the 15-nation council to recognise the Zimbabwe opposition’s claim to power, based on the results of the March 29 first round. Instead, the watered-down Security Council text said merely that “the results of the 29 March 2008 elections must be respected”.

Ban Ki Moon, the UN Secretary-General, called for Friday’s presidential run-off in Zimbabwe to be postponed because of violence and intimidation. “There has been too much violence, too much intimidation. A vote held in these conditions would lack all legitimacy,” he said. Nelson Mandela, the former South African President, arrived in Britain yesterday amid increasing pressure for his country to exert its influence on the Mugabe regime - and for him personally to speak out against Mr Mugabe.

There were cross-party calls in the Commons for Mr Mandela - who meets Mr Brown today - to use his moral authority. But Downing Street sought privately to make excuses for Mr Mandela’s silence and played down the prospect of him intervening.

President Mbeki of South Africa is expected in Harare tomorrow for a final attempt to push Mr Mugabe into negotiations for a settlement with the MDC, diplomatic sources said. It is his second visit in a week and follows a reported rebuff by Mr Mugabe of his proposals for a government of national unity. Mr Tsvangirai said he would consider talks with Mr Mugabe over a national unity government only if the violence was halted.

Lord Malloch-Brown, the Foreign Office Minister, said that military action was “not a plausible course” and “would not enjoy international support”. However, there might be support for the use of international peacekeepers from African Union countries, he said.

Britain would work for a “deepening” of existing EU sanctions against 130 leading figures in the Zanu (PF) regime including possibly banning their children from studying abroad. The EU could also look at tightening the restrictions on international firms doing business with Zimbabwe.

The Government would also review British companies doing business in Zimbabwe. The four remaining major UK companies in the country are British American Tobacco (BAT), Barclays Bank and the miners Rio Tinto and Anglo American.

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