IT WAS intended to be the symbolic gesture at a global series of rock concerts next month to alert people to climate change. Al Gore, the former US presidential candidate turned climate doomsayer, had wanted a massive switch-off of lights by television audiences, but the National Grid has vetoed the idea.

The inconvenient truth, it says, is that the power surge when people switched their lights back on could cause disruptions in supply and even endanger hospital patients on life support machines.

Live Earth will be a series of concerts, modelled on Live Aid and and Live 8, aimed at raising awareness about the threat from global warming.

As many as 12 concerts across seven continents featuring the likes of Madonna and Genesis and 100 other acts are planned over 24 hours, including one at a research station in Antarctica.

The organisers have so far struggled to find a clear-cut way of conveying their main message. Even rock performers have criticised the concept.

Roger Daltrey, of the Who, said another concert would simply waste fuel; Bob Geldof, who helped to organise Live Aid and Live 8, said people were already aware of the greenhouse effect; while Matt Bellamy, front man of the rock band Muse, labelled it “private jets for climate change”.

The switch-off was conceived as an emblematic act in the same way that Will Smith, the actor, coordinated people across the world to click their fingers every three seconds during the Live 8 concerts to convey that in Africa another child had died.

It was meant to create a moment that would resonate round the world and provide a counterpoint to the old fad for holding carbon-emitting cigarette lighters aloft at concerts.

It would also have given Britain its biggest blackout since the blitz and the miners’ strikes of the 1970s – and encapsulated the message of the urgency to save energy.

However, it has had to be shelved after the keepers of Britain’s power supply said no. “We are in favour of sustained energy efficiency as opposed to people just doing it very suddenly as a stunt,” said a spokesman for the National Grid.

“The organisers of Live Earth planned to do this very symbolic act but we had concerns because it was impossible for us to forecast what would happen.”

John Gaydon, producer of the British concert at the new Wembley stadium, said: “The National Grid warned us that it would put too much pressure on the power supply and would be potentially dangerous for hospitals.”

Gore has admitted that the concerts will consume a vast amount of electricity. To combat criticism of their own damaging effect on the environment, the organisers will pay at least £1m in carbon credits and supply acts with hybrid cars, partly run on electricity, to ferry stars to venues as well as fuel-efficient Smart cars to run around backstage.

They also claim that the power for the shows will come from green suppliers and biofuels. Food and drink sellers will use biodegradable plastics and there will be waste recycling systems.

The event at Wembley on July 7 will be headlined by Madonna, who green campaigners say used 440 tons of CO2 in four months on her Confessions tour last year. Artists from America lined up for the stadium include the Black Eyed Peas, Red Hot Chili Peppers and John Legend, a singer who has been criticised for making advertisements for 4x4 vehicles.

A proposal for Gore to appear at concerts in Britain and America on the same day – something that Phil Collins, the Genesis drummer and singer, was able to do at the original Live Aid in 1985 courtesy of Concorde – has been dropped because of the anger that the “gas-guzzling” flight would provoke among environmentalists.

The National Grid intervention puts in doubt whether Gore, whose surprise hit film An Inconvenient Truth warned of the imminent dangers of global warming, will appear in London at all. A spokesman for his office said: “We are not entirely sure where he is going to be. It would have been logical for him to have been in London but now he is probably going to be in New York at some point.”

The New York show will include British stars such as the Police and Roger Waters. A global TV audience of two billion is expected to watch the events. There will also be concerts in Brazil, China, Japan, South Africa, Australia and Turkey.

Janet Wood, of Utility Week, the industry’s trade journal, said: “Power companies spend a lot of time predicting when they are going to have high demands.

“They can cope with sudden surges when they know they are going to happen such as at the end of a television programme, but they like to know exactly when they are going to happen.

“The organisers of the concerts might decide to buy from green suppliers but they have no way of knowing how the electrons they use are generated.”

The biggest recorded power drop in Britain was 2,700 megawatts during the three-minute silence for the victims of the September 11, 2001 attacks. The biggest surge was 3,000 megawatts after the solar eclipse in 1999.

Deprived of their concept of creating a mass blackout, the Live Earth organisers will rely instead on the spoof rock band Spinal Tap to help the public see the light on global warming.

The band, whose guitarist Nigel Tufnel notoriously insisted on turning his speakers beyond the maximum 10 to 11, will be joined on stage by Ricky Gervais, the comedian.

Rob Reiner, director of the film which launched Spinal Tap in 1984, said that in the routine for Live Earth the band initially struggles to grasp the concept of climate change. “Nigel thought it was because he was wearing too much clothing – and that if he just took his jacket off it would be cooler,” Reiner said.

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