Her decision follows the announcement on Wednesday that Des O'Connor, the 76-year-old host of Countdown, is also to quit the Leeds-based words and numbers quiz show .

Vorderman, 47, has been with the show since it began in 1982 and was the first woman to appear on Channel 4.

The presenter, who appeared on almost 5,000 episodes of the show, was said to be "extremely sad".

It is believed Vorderman's decision was motivated by a desire not to go through having to bond with yet another new co-presenter.

Her manager, John Miles, said: "She didn't think she could go through that process again. She just feels that it is time to leave.

"Carol was the first woman on Channel 4 and she is enormously grateful to have had 26 wonderful years and nearly 5,000 shows on Countdown.

"She wants to thank all of the crew, the thousands of contestants who have played the game and the millions of Countdown viewers for whom she has the greatest respect."

Vorderman has been a mainstay of Countdown since making her debut on its first ever show, coming into her own as the show's resident maths expert.

She even became something of an unexpected sex symbol among male fans, many of whom viewed her as a thinking man's pin-up.

For more than 20 years she acted as a foil to the show's main presenter Richard Whiteley, who died in 2005.

In a statement, her manager said she had considered quitting following his death, but eventually agreed to continue alongside first Des Lynam and then Des O'Connor. She is step down at the end of the year.

Earlier this week, O'Connor announced his decision to step down from presenting the weekday show in November.

Bookmaker William Hill has stand-up comic Tom O'Connor as favourite at 8/1, with comic Jo Brand at 10/1, prog rocker Rick Wakeman at 20/1 and motormouth Top Gear frontman Jeremy Clarkson at 33/1 to move into the hot seat.

Less obvious candidates include Yorkshire cricket legend Geoff Boycott (150/1), reformed rock 'n' roll hellraiser Ozzy Osbourne (200/1) and ex-PM Tony Blair (500/1).

A spokeswoman for Channel 4 said an announcement on a new presenter would be made "in due course".

Des O'Connor is stepping down because he misses the excitement of his one-man concerts. The 76-year-old all-round entertainer is also in talks about a part in a major West End stage production.

He has been anchoring Countdown since late 2006, when he succeeded housewives' favourite Des Lynam in the presenter's chair.

Lynam had taken over in 2005 following the death of the programme's only previous host, inimitable Yorkshireman Richard Whiteley.

Countdown, which is filmed at the Yorkshire Television studios on Kirkstall Road in Leeds, was the first show broadcast on Channel 4 when the station launched in 1982.