"Moonlighting" by MPs must end, Gordon Brown is being urged, in a drive to clean up British politics.
One in four MPs, including nearly half all Conservatives, have highly paid directorships that keep them away from Westminster, new figures show.
The scale of politicians' part-time work alarms Downing Street because it risks fuelling criticism over MPs' expenses.
Several members of David Cameron's shadow cabinet are among 96 Tory MPs who record non-political employment on the latest register of interests. There are 39 Labour MPs, 14 Liberal Democrats and eight from minor parties also engaged in outside jobs.
Some privately defend the extra work, arguing they need to supplement their £61,820 salaries, while others say the business experience is vital to a politician.
But there are fears that public disillusionment over expenses could spread to other aspects of MPs' work, so advisers are urging the Prime Minister to curb the part-time activities.
The former health secretary Patricia Hewitt has two "advisory" roles, with private equity firm Cinven and Alliance Boots. She takes up a non executive directorship at BT, tomorrow, bringing her total annual earnings to about £170,000.
Her ex-cabinet colleague Alan Milburn is paid up to £30,000 as an adviser to Lloydspharmacy, £35,000 to advise Bridgepoint Capital and £25,000 by PepsiCo.
Labour MP Martin Salter, who tabled a Bill last year calling for an end to "moonlighting", said: "This is one of the unpublicised scandals in Parliament.
"There is a minority of MPs who treat representing 70,000 constituents as a part-time job, when they are receiving a full-time salary from the taxpayer."