The 13 "house guests" will compete to win $500,000

The show launched on the CBS network on Tuesday and was the most popular programme during its timeslot.

Eight contestants entered the house first - only to realise it was equipped for 13 people and five of their former loves were to join them.

But Big Brother has not proved a huge success in the US, while other reality shows like Survivor Joe Millionaire have pulled audiences of up to 52 million.

An average of 9.7 million people watched Tuesday's show, beating Simon Fuller's latest talent show, American Juniors, and another European import, game show Dog Eat Dog.

'X factor'

The 13 "house guests" will compete to stay in the house for 100 days and win $500,000 (�300,000). They will be constantly filmed and allowed no contact with the outside world.

The show's website claims the "X factor" is "turning up the heat in the house" with some former couples constantly arguing and others getting reacquainted.

This year's Big Brother contestants include a 58-year-old retired FBI agent, a karate school manager and a head-hunter.

An added twist is a new post, Head of Household - which allows one person to get a private room with a lockable door, laundry service and a stocked fridge.

UK ratings down

One contestant will be voted out every Wednesday.

Big Brother has been a hit around the world since launching in the Netherlands in 1999, with the UK's fourth series well under way.

Ratings have settled down, with about 4.5 million people tuning in for each nightly update.