Mr Howard, 57, has had trouble living down Ann Widdecombe's claim that there was "something of the night" about him.
Mr Portillo, a former Tory cabinet minister who lost his seat at the last general election, said on BBC radio yesterday: "He has obviously had, let's be frank, an image problem. He may well feel he can do more good for the Tory party from the back benches and doing work on policies ... than he is now able to do on the front benches."
Mr Howard said that 14 years on the front benches was "probably long enough for anyone". But Margaret Becket, Leader of the House, said: "Michael Howard's personal unpopularity is a convenient excuse for Tories unwilling to face up to the reality of their party's failings."
His departure will be used by William Hague, the Tory leader, to cut out some of the dead wood from the Shadow Cabinet in his next reshuffle to answer criticism in the party's private polling that the Conservatives are still seen as remote, out of touch, and out of date.
Ms Widdecombe, who effectively killed Mr Howard's leadership prospects, is likely to gain promotion from the health portfolio to become home affairs spokeswoman, replacing Sir Norman Fowler, who could also leave the front bench this summer.
Mr Howard's decision to resign from the Shadow Cabinet, while staying on as MP for Folkestone and Hythe, was also seen at Westminster as an admission that he does not believe the Tories can win the next general election under Mr Hague.
Mr Hague's failure to make an impact on Tony Blair's lead in the opinion polls has reinforced the conviction among leading Tories that fresh faces are urgently needed to give the party a new start.
Those being tipped for promotion include Theresa May, aneducation spokeswoman who has been compared to a young Margaret Thatcher; Richard Ottaway, who changed Tory policy to support the post for an elected mayor for London; Andrew Lansley, the former head of research, who is leading a review of policy for the general election; Cheryl Gillan, a spokeswoman on foreign affairs praised for being a safe pair of hands; Phil Hammond, a talented spokes-man on health in Ms Widdecombe's team; and Damian Green, a rising star in the education and employment team well thought of by Mr Hague.
Despite the criticism, Mr Howard, one of the few big hitters on the Tory front bench, will be a big loss to Mr Hague's team. The foreign affairs spokesman's shoes will be difficult to fill, given the sensitivity of handling the issue of Europe. John Maples, the former health minister, is being tipped as a possible contender.
Another former cabinet colleague, Sir Brian Mawhinney, said: "He's made Robin Cook look like one of the more pathetic Foreign Secretaries that this country has had in many years."
Mr Hague is planning to use a speech to his party's spring forum in Reading at the weekend to draw a line under the Tories' 18-year record in office and move the Conservatives on to "bread and butter" issues.
"We are not talking about apologising for our 18 years in office. It's not about apologising. We had some successes and we made some mistakes. But it's about moving on to a new agenda," he has said.
The Rising Stars
Andrew Lansley: Leading a review of policy for the general election
Phil Hammond: Considered to be talented spokesman on health for the party
Richard Ottaway: Changed party policy to support an elected mayor for London
Theresa May: Has been compared to a young Margaret Thatcher
Damian Green: Influential member of the education and employment team
Cheryl Gillan: A foreign affairs spokeswoman and seen as a safe pair of handsReuse content