Portillo recently spent a televised week as a single mum

The Kensington and Chelsea MP said new Conservative leader Michael Howard had offered him a shadow job, which "brought things to a head".

He could not turn down the offer without explaining his decision to leave parliament and so had brought forward his announcement, he said.

In a statement, Mr Portillo said: "I've been considering my future for some time and I've decided my future lies outside the House of Commons.

"In a number of ways I have lost my enthusiasm for the cut and thrust of the chamber and my contribution has diminished.

"I don't know what I'm going to do, I am keen to explore opportunities in the media, public bodies and the arts."

Howard's sadness

Mr Portillo has already built up a portfolio of media work and recently spent a week as a single mother for a TV documentary.

He first entered Parliament in 1984 but famously lost his Enfield Southgate seat in the Tories' 1997 landslide election defeat.

He returned in the by-election which followed MP Alan Clark's death in 1999, serving as shadow chancellor under William Hague and pushing an agenda of modernisation and tolerance.

His friends have long believed he was deeply ambivalent about his future in the Conservative party Analysis: End of a once golden career

Mr Portillo said he would probably have announced his decision in a couple of weeks time in any case

He would not say what job he had turned down but said there was absolutely nothing wrong with the offer.

"I wanted it to be clear that the only reason I wasn't serving in the shadow cabinet was because I'm not going to be in the next Parliament and certainly not because I have any objection or lack of enthusiasm for working with Michael Howard," he told BBC News 24.

Career change

Accusations that he and his supporters were plotting for the leadership had made him fed up, he said, but he also thought he had become less partisan.

His time on the back benches had allowed him to explore other careers, he said.

Bercow: Praised dignity in adversity

"Many people have lots of careers in their life times and that goes by more or less without comment," Mr Portillo said.

Mr Portillo said he had been thinking about his future since the 2001 election but had not received any job offers outside Parliament.

"This is really jumping off a high board into a swimming pool that I hope has got some water in it," he said.

'Popular MP'

Mr Howard said he was "extremely sad" about Mr Portillo's decision.

In a letter to Mr Portillo, he said: "Over more than 20 years you have made an immense contribution both to the Conservative Party and to our country.

"We are all enormously grateful. Your departure from the House of Commons will be a great loss."

Shireen Ritchie, chairman of the Kensington and Chelsea Conservative Association, said Mr Portillo had told her he was standing down in a letter on Thursday.

She said: "Michael has been open with me and I am aware that he has been giving serious thought to his future for some time...

"Michael has been an extremely assiduous and popular member in this constituency."

Labour attack

John Bercow, who resigned as a shadow minister to vote with Mr Portillo in favour of allowing unmarried couples to adopt said he was sorry to hear Friday's news.

"Michael was an excellent minister," he said. "He has shown dignity in adversity and he made us all think about how the Conservative Party must change to regain mass public support."

Andrew Mackay, a close ally of Mr Portillo, said: "I have always felt he had a major role to play in British politics for a long time to come... "It is a sad day for British politics and a sad day for the Conservative Party."

But Health Secretary John Reid seized on Mr Portillo's decision, saying: "This is a massive snub to Mr Poll Tax - Michael Howard. It shows the moderates and modernisers in the Conservative Party have given up."