Nobody has owned up to it yet but fortunately it happened during the second interval

San Francisco Ballet spokesman

But fire-fighters traced the cause of the alert to a backstage microwave, where a baked potato had been forgotten by a dancer, and exploded.

"The microwave seemed fine but there wasn't much left of the potato," a spokesman for London Fire Brigade said.

The incident happened during the second interval of the company's show at the Covent Garden venue.

"There were wisps of smoke. It took fire-fighters a while to find the cause of the smoke," the spokesman said.

The Opera House was rebuilt with donations

"Eventually they found a small kitchenette backstage and in a microwave were the remains of an exploded potato."

A spokeswoman for the San Francisco Ballet, which is performing at the venue this week, said nobody has owned up to cooking the potato.

"Fortunately it happened during the second interval," she said.

"The audience waited for around 20 minutes outside, which was the length of the break. They only missed out on their drinks."

The Royal Opera House is bracing itself for more serious criticism from Dame Vivien Duffield, who is leaving after the venue decided to merge her fundraising trust with another body.

'Tireless efforts'

She is credited with donating up to �25m from her personal fortune, and raising a further �100m as chairman of the Royal Opera House Trust.

The venue has confirmed that the Trust will be merged with The Friends of Covent Garden to create The Royal Opera House Foundation.

Sir Colin Southgate, the venue's chairman, praised Dame Vivien's "tireless efforts and invaluable contribution", which he said are unlikely to ever be surpassed.

Her contribution was instrumental in the rebuilding of the venue in the 1990s.