The  British Government will call for Zimbabwe's power supplies to be cut off if president Robert Mugabe rigs the election, it emerged.

Diplomats and allies are urging South Africa to block electricity lines to the country amid fears Mr Mugabe will cling to office.

The 84-year-old ruler has threatened violence if the opposition party wins the election run-off on 27 June.

Zimbabwe president Robert Mugabe attending a funeral of a general at the weekend. He called the opposition movement 'pathetic puppets'

He warned last week that supporters of the ruling Zanu-PF party will take up arms to stop the Movement for Democratic Change (MDC) taking over.

Fears escalated further after he openly vowed that the MDC will 'never rule Zimbabwe' despite gaining almost 50 per cent of the vote in the first elections in March.

Now urgent plans for sanctions are being drawn up to head off a diplomatic and humanitarian crisis. Diplomats are considering a ban on the children of Zimbabwe's elite going to school in Europe if Mr Mugabe loses but refuses to step down.

Bank accounts and assets held in the US and Europe could be frozen and all aid could also be halted.

The Foreign and Commonwealth Office is also hoping to persuade Zimbabwe's neighbours to create an economic blockade.

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Some of Zimbabwe's electricity comes from South Africa and diplomats believe they might be able to persuade the South African government to restrict or turn off the supply. Vital imports also come through Mozambique and South Africa.

One diplomat said: 'One way or another, this summer is likely to mark the endgame for Robert Mugabe.'

Morgan Tsvangirai, leader of the MDC, won the first round of elections in March but with slightly fewer votes than he needed to secure the presidency.

The Mugabe regime has since unleashed thugs, war veterans and police to intimidate and beat up opposition supporters before the second poll.

At least 66 MDC supporters have reportedly been killed, Mr Tsvangirai has been arrested five times, and his senior aide Tendai Biti was charged with treason.

Mr Mugabe said over the weekend: 'We shall never, never accept anything that smells of ... the MDC. These pathetic puppets taking over this country? Let's see. That is not going to happen.'

'We are prepared to fight for if we lose it in the same way that our forefathers lost it. We are prepared to fight for our country and to go to war if we lose it the same way our ancestors lost it.'

Foreign secretary David Miliband described his actions as 'sadism' and lobbied UN secretary general Ban Ki-Moon to take action.

A third of Zimbabwe's four million people are reliant on food aid yet the regime has banned relief organisations from distributing supplies.

Political violence has spread from the countryside to urban areas around the capital Harare. Some opposition supporters today complained of being attacked in a township near the city.