"I can confirm we have been told that Humphrey sadly died last week some time," a Downing Street spokesman said.

'Chief Mouser'

Official questions were asked in the House of Commons about Humphrey's absence after his departure from Downing Street in 1997.

A Conservative MP demanded proof that the moggy - sometimes given the nickname Chief Mouser to the Cabinet Office - was still alive.

Downing Street said Humphrey had been suffering from a kidney complaint.

The media were later invited to a South London property, where Humphrey was photographed, hostage-style, with copies of the day's newspapers.

He had also gone missing while John Major was in Downing Street.

At the time it was thought he had died.

But he returned after staff at a Westminster medical college read his obituary and realised the cat that had made his way into their building was, in fact, Humphrey.

'State of shock'

Participants in the programme, called A Challenge to the Heart, raise funds for local charities by completing difficult tasks set by the network - in this case manoeuvring a train a certain distance down railway tracks.

The Associated Press news agency quoted Ana Portela as telling local radio station El Espectador that the train was moving when "somebody slipped and fell under the locomotive, and others were falling alongside it.

"There were shouts and somebody said 'my arm!'" Ms Portela said.

"Everybody was in a state of shock."

Graciela Baccino, a spokeswoman for Channel 10 television, said the tragedy was "deeply saddening".

She said the TV network offered its "solidarity and support" to the families involved.

About 3,000 schoolchildren were at the recording and witnessed the accident.