Oliver Letwin, the Conservatives’ policy chief, is paid an annual salary of £60,000 by an investment bank for working eight hours a week, The Times has learnt.

N M Rothschild Corporate Finance Ltd disclosed his £145-an-hour earnings before a change to parliamentary rules which require MPs to detail the time they spend on outside jobs.

Mr Letwin, who is writing the party’s manifesto, approved the release of the information in a sign of growing Tory defiance over the issue of MPs’ outside employment.

William Hague and Ken Clarke are among other senior Tories holding on to their second jobs despite the new requirement to disclose how much of their working week is spent topping up their MPs’ salaries. Others, like Andrew Mitchell, the Shadow International Development Secretary, are giving up only some of their well-paid roles outside Parliament.

The requirement on MPs to list hours spent on second jobs was introduced as part of reforms prompted by the expenses row. The Tories initially resisted the change, claiming it would prove embarrassing for the party to disclose hourly rates.

A spokesman for the Tory leader last night insisted that he was “perfectly relaxed” about his most senior colleagues earning extra cash outside Parliament ahead of the general election. He has told members of the Shadow Cabinet — but not other frontbenchers — that they should ditch all other jobs early next year, however.

Mr Cameron clearly hopes that that commitment, together with the fact that many former Labour ministers also earn significant sums outside Parliament, will limit the damage to the party when the full details are published in future editions of the register of MPs’ interests.

A number of MPs, already bruised by the expenses row, are giving up their second jobs before the July 1 deadline. Ian McCartney has become the first former minister to give up a lucrative private-sector job.

The former Trade and Industry Minister has announced that he will resign from his £113,000-a-year job as a consultant for an American nuclear energy operator.

His move will put pressure on at least 20 former members of the Government who have taken second jobs in the private sector after losing their ministerial positions. They are estimated to be earning a total of at least £2 million a year through companies often closely linked with their former Government jobs.

Since 2006 37 members of the Government have been given permission to accept private sector jobs within two years of leaving office. The Advisory Committee on Business Appointments has approved the jobs for 20 sitting MPs, 16 Peers and Tony Blair, the former Prime Minister.

Mr McCartney will resign on June 30, meaning that he will not have to reveal how much he is paid or the amount of time he spends working for Fluor Corporation under rules that come into force the next day.

Mr McCartney started advising Fluor Corporation within months of leaving the Department of Trade and Industry, which was responsible for Britain’s nuclear power strategy, during the Cabinet reshuffle in June 2007. In November 2007 he became embroiled in the “cash for access” controversy, when he was seen entertaining a senior executive from Fluor in the Commons. He insisted that it was a social meeting.

In his entry in the register of MPs’ interests published on June 10 the former Labour Party Chairman told the Commons authorities that he had “taken no income personally” from his Fluor remunerations. He said that all the money from the company went to pay a researcher or to charity.

The edition published in May, however, does not make this claim, only that he paid for a researcher from the money paid to him by the company.

Mr McCartney announced last month that he would stand down at the next election because of “health problems”, days after his disclosure that he paid back almost £15,000 worth of expenses claims last year.

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