Torrential rain delayed the start of the Glastonbury Festival this morning, flooding fans' tents and forcing organisers to switch off power as a wave of thunderstorms swept up from the south-west.

Bands were due to take the various stages at 10.30am, but organisers were forced to postpone the festival for some two hours while they sorted out safety issues.

"Two inches of rain in just a few hours mean that the festival will be a muddy one from now on but the forecast is good from now on," said Michael Eavis, the festival organiser. "We get one of these about every five years but it won’t cause any major problems."

The annual Somerset rockfest was the most high-profile casualty of a wave of storms moving in from the west and bringing a sudden end to an early-summer heatwave in England and Wales. The West Country and Midlands bore the brunt, with flooding reported in Torquay elsewhere in Devon and even a tornado in Coventry.

Paul Simons, Times weatherman, said the storms were caused by a front of cold air sweeping in from the west and colliding with hot, muggy air moving up from the Continent. "Where the two meet, it's detonated huge thunderstorms," he said. "We're expecting the thunderstorms to carry on moving eastwards and probably hit London some time this afternoon."

As with the Wimbledon tennis championships, Glastonbury had basked in glorious sunshine throughout the week. But tens of thousands of revellers were woken up by the roar of thunder at dawn, and the storms soon turned the site into a mudbath.

BBC Radio 1’s live coverage was abandoned after a river, close to the studio being used by DJ Jo Whiley, rose by a foot in just 15 minutes.The weather cut the power to the main Pyramid Stage, which is due to be headlined tonight by the White Stripes. Lightning also struck a beer tent in the festival's 'dance field', although it was not in use at the time.

Dafydd Goff, a music journalist who is writing a Times Online weblog from the festival, said he was enjoying a cider-induced sleep when the storms hit, and six and a half hours of rain began.

"It looks like one big paddy field," Goff said. "This is the most torrential I've ever seen it at Glastonbury. People are singing to try to raise spirits, but eveything's flooded out."

Tens of thousands of campers sheltered in their tents and caravans as lightning flashed and thunder echoed around the giant site. Only a few brave festival-goers ventured into the downpour to trudge through the mud in search of breakfast.

It was a far cry from yesterday when hordes of people arrived at the festival - which has a reputation for being a wash-out - in blazing sunshine. Today they swapped their bikinis for raincoats and bin liners and took to huddling in the cafe tents dotted around the site at Worthy Farm in Pilton.

Jeremy Knight, 46, from Bedford, who is on his first trip to Glastonbury, came prepared in a cagoule and wellies. "I don’t think you can come to Glastonbury and be too disappointed if it rains," he said. "It’s a bit like Wimbledon - of course it’s going to rain."

Also smiling was the owner of a camping equipment stall near the main stage, who said: "We love the rain. We sold out of wellies hours ago, and the raincoats will be gone soon."

Anton Muscat, a national forecaster at the Met Office, said the storms began in Cornwall and Devon in the early hours and were gradually heading east and north. The Midlands took the brunt of most of the rain but the band of storms was stretching from Oxford down to Chichester in Sussex and was expected to reach London by mid-afternoon.

Mr Muscat said the storms would provide some relief for southern areas most badly hit by the recent drought, although much of the rain would soon wash away.

But health officials say the storms could unleash major problems among those prone to respiratory illness, especially asthma and hayfever sufferers.

According to the Met Office health forecasting unit, the danger comes from a rare mix of severe ozone pollution, hot temperatures, high levels of pollen and violent thunderstorms. A similar combination in London on June 24, 1994, caused thousands of asthma cases that overwhelmed hospitals and clinics.

Extremely powerful thunderstorms can trigger asthma attacks by sucking up huge amounts of air and pollen from the ground. Violent updraughts in the thunderclouds shoot the pollen grains high and shatter them in the freezing conditions at the top of the clouds.

Cold air loaded with the pollen debris then plunges down and floods the ground with a vast dose of pollen particles, which are easily breathed deep into the lungs and set off a violent allergic reaction in hayfever sufferers. With high levels of ozone already breathed in during the day, the extra assault to the body becomes even more dangerous.

William Bird, clinical director of health forecasting at the Met Office, said: "The people at risk have never had asthma before. They have no medication, they’re terrified."

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