Michael Portillo was facing an official complaint last night that he should have declared his links with an oil company when he spoke in the House of Commons about petrol duty.
Chris Leslie, Labour MP for Shipley, wrote to the Parliamentary commissioner for standards to ask for an inquiry into the shadow Chancellor's job with the Kerr McGee Corporation. Yesterday, he said that opposition front-benchers should, like government ministers, be prohibited from holding directorships.
Many members of the Shadow Cabinet have outside jobs, some of which relate directly to their portfolios.
Last year the commissioner, Elizabeth Filkin, found that Mr Portillo's predecessor, Francis Maude, had unintentionally broken the rules when he proposed a new clause for the Government's Finance Bill relating to the retail industry. At the time Mr Maude was a director of Asda.
"There is a lot of money to be made out there and they are obviously not shy about taking it," Mr Leslie said. "I happen to think it is wrong."
Mr Portillo declares in the Register of Members Interests that he is paid up to £10,000 per year as an adviser to Kerr McGee, the world's third biggest oil exploration company, and up to £15,000 as an adviser to the Probyn Group, which finances renewable energy projects.
On March 27, in the Budget debate, he pointed out that the level of inflation by which petrol duty was raised was higher than that used for other taxes.
MPs are allowed to have outside interests and to speak about them in debates, but they must make a verbal declaration of their interest.