Michael Portillo has announced that he is to stand down from the House of Commons at the next election.

The move is being seen as a blow to Michael Howard as he seeks to regroup top Tories following his election as leader.

Portillo's decision effectively ends one of the most high profile Conservative careers of recent times.

The former defence secretary had been tipped as a possible member of Howard's new Shadow Cabinet.

Portillo confirmed he had been offered a new post but declined to rejoin the frontbench.

"I have been considering my future for some time and I have decided that it should lie outside the House of Commons," he said.

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

"I do not know what I shall do after I leave, but I am keen to explore opportunities in the media, public bodies and the arts.

"Michael Howard offered me a job in the shadow cabinet, he tried to persuade me to stay, but I would not change my mind."

Responding to Portillo's announcement, Howard said: "I am extremely sad that you have decided not to stand again at the next election.

"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."

Speaking earlier this week, Portillo said he believed Howard's victory effectively meant that modernisers had lost the argument.

The one-time right-winger lost his Enfield Southgate seat in the 1997 general election after a huge swing to Labour.

After a sabbatical in which he went back to his Spanish roots, he re-entered the Commons in a by-election following the death of Alan Clark.

Since returning to parliament many believe Portillo failed to rediscover the sense of purpose which propelled him through the ranks of the Thatcher and Major governments.

His admission to a homosexual past and his subsequent conversion to moderate Toryism damaged him in the eyes of many traditional Conservative supporters.

Whilst he was popular in the country, many MPs never forgave him for the behind-the-scenes briefings which undermined William Hague's leadership.

The chairman of the Kensington and Chelsea Conservative Association said he had been a "very loyal and supportive" MP.

"We have enjoyed having Michael as our member of parliament and he will be much missed in Kensington and Chelsea," said Shireen Ritchie.

"He retains our affection and both he and Carolyn have our very best wishes for the future."

Portillo said he would continue to work for his constituents until the next election.

But he added that the time had come to seek a life outside Westminster.

"I may not be the only MP to whom that has happened over a period of time, I think people do change and I think it is better to admit that change has happened," he said.

"People outside public life do have different careers. I hope it is true that I have remained very committed to my constituency and that I have done a good job."