David Cameron announced today that all of his shadow ministers would give up their second jobs by the end of the year.
The Tory leader dismissed any suggestion that outside interests were incompatible with being a good MP.
But he added: "My shadow cabinet have, however, recognised that we are in a particular period at the end of a five-year Parliament where it does become necessary to demonstrate 100% focus on Parliament, politics and setting out our credentials as an alternative government.
"So they've decided that from the end of December they won't have any outside interests."
He said that all of his shadow spokespeople in the Commons, and shadow leader of the Lords Lord Strathclyde, had agreed the step.
The Tories were today publishing details of their shadow cabinet's current outside interests, Mr Cameron told a Westminster press conference.
Reforms coming into force from Wednesday mean all MPs will be obliged to list hours and details of salary.
But the Tory leader said his shadow ministers wanted to publish their information early and at the same time.
"We all want this information to be available now, not to emerge in a fragmented way, as and when individual declarations are made," he said.
Previously, MPs have had to name their outside employers and directorships in the Register of Members' Interests.
But they have never before been forced to reveal how long they spend on the work and only needed to give an indication of how much they were paid if the job related to their work as an MP.
More than half of the MPs with outside interests are Conservatives, but many have already announced they are giving their second jobs up ahead of the changes.
Some high-earning Tory frontbenchers, including shadow foreign secretary William Hague, have already indicated that they are to quit outside roles.
Mr Hague last year earned about £230,000 from after-dinner speeches, advice to private companies and writing books.
Other senior Tories quitting their outside positions include policy chief Oliver Letwin, who earns a £60,000 salary for eight hours a week at investment bank NM Rothschild, and shadow skills secretary David Willetts, who is paid £80,000 a year to advise a pensions company.
Shadow Commons leader Alan Duncan has already given up directorships of two oil companies and a US-based engineering company, from which he earned more than £80,000 a year.
Mr Cameron said that, in general, he felt that MPs having outside interests could be "a good thing" as long as there were "proper rules" for disclosure and banned advocacy.
"I do not think that a chamber full of professional politicians with no outside experience is a good thing," he said.
"MPs and members of the Shadow Cabinet should be judged by what they do for their constituents and holding the Government to account.
"There are idle MPs with no outside interests and there are fantastic public servants that do have them."