The commercial network is claiming an estimated 17m viewers watched Sunday night's edition of Chris Tarrant's quiz show Who Wants To Be A Millionaire?

If the provisional figures are correct, the programme achieved the highest audience for a TV show since 1997, excluding soaps and the World Cup.

But even though nobody has yet won the top prize, it has not stopped viewers calling in to try to enter the show. At least 1.7m have called a premium rate number to take part in this current series, which is running nightly on ITV until January 12.

Together with the 3.1m who called a premium rate number to take part in the first series last year, it has added up to a reported £2m income for Celador Productions and ITV, with the average cost of each call put at 77p.

BT also reaped a £1.7m bonus for renting out the line, according to the Daily Telegraph.

Celador disputes the figures, but a spokeswoman for the programme said the cash generated by the phone line went back into the show's prize fund.

"The money helps pay for the production and promotional costs of the show, and it also goes into the prize fund. It will also help pay for a third series of the show if it is recommissioned by ITV.

"If ITV don't recommission the programme, we'll give what's left to charity."

Coronation Street: Helped ITV to a Sunday night ratings victory

Although nobody has yet won more than £64,000 on the programme, she insisted the phone line wasn't just an easy way of making money.

"We're taking quite a risk with the show - we could have a string of millionaire winners in a row," she said.

"It's very good TV - even if the contestants don't win a million pounds they can still win a big sum like £12,000. And there's always a fallback option so contestants don't lose all their winnings if they lose in the quiz."

Together with an estimated 17.6m who watched Coronation Street, ITV is claiming a 62% share of the Sunday night audience - its best performance in almost a year.

The prizes on offer may be huge, but the idea behind the show is simple. Contestants have to answer 15 multiple-choice questions of varying difficulty to win the top prize of £1m.

Yet they have three chances to gain help along the way. They can either ask the audience for help, call a friend or remove two of the wrong answers from the options.

In offering the top prize, the show takes advantage of the liberalisation of commercial television regulations in 1993.

Before that, the top prize available on TV was strictly regulated - with a general rule that the prize should be no more valuable than a small family car.

Host Chris Tarrant suspects it isn't just the huge prize that is behind the show's success.