Nelson Mandela will speak out about the situation in Zimbabwe tonight, The Times has learnt.

The former South African president will use a fundraising dinner to make public his concerns about the worsening crisis in the country, to an audience that will include the Prime Minister, Bill Clinton, the Mayor of London and most of the artists who will perform at his 90th birthday concert on Friday.

A source close to Mr Mandela said that he made the decision to raise the issue during his visit to London when he flew into the country on Monday morning, to learn that Morgan Tsvangirai, leader of the opposition MDC party, had pulled out of Friday’s run-off election hours earlier.

“When he got off the plane it was clear that the situation had changed dramatically while he was in the air,” the source said.

However, Mr Mandela felt he had to wait for the UN to take the initiative and for Robert Mugabe to confirm his intention to go ahead with the election despite the absence of a challenger before making his statement.

Earlier this week the UN security council unanimously condemned the violence in Zimbabwe, while the UN secretary general, Ban ki-Moon, said going ahead with the run-off would “deepen divisions within the country and produce a result that cannot be credible”. Yesterday Robert Mugabe vowed defiantly to proceed with the election.

It is understood that Mr Mandela will issue a condemnation of the chaos engulfing Zimbabwe, though he will not call for any specific action to be taken against Mr Mugabe.

But the words of such a universally respected figure will carry weight around the world. “Whatever he says, people will act upon,” the source said.

His statement will also intensify the pressure on the South African president, Thabo Mbeki, who has so far avoided public criticism of Mr Mugabe, and will be taken as a sign that Mr Mandela believes the policy of quiet diplomacy has run its course.

Mr Mandela’s camp is believed to be irritated with reports that the Prime Minister urged the 89-year-old to make a statement on Zimbabwe at their meeting on Monday. “It had nothing to do with the Prime Minister. He didn’t speak to him about it,” the source said.

Earlier today Mr Mandela was granted an audience with the Queen. Despite looking frail, and walking with the aid of a stick, the world’s favourite statesman was on typically mischievous form, telling the monarch: “you look younger every time I see you.”

The Queen received him with every sign of enjoying the chance to renew the friendship – despite the clash between the ornate décor of the royal residence at Buckingham Palace and the Mr Mandela’s distinctively patterned shirt.

“It’s very nice weather, arranged just for you,” she joked in front of the cameras as they sat down, before an informal private chat.

The private dinner in Hyde Park this evening will be attended by many of the celebrities who act as ambassadors for Mr Mandela’s charities. They will return there on Friday for a charity concert featuring performers including Queen, Leona Lewis, Joan Baez, Razorlight and Annie Lennox.

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