After expenses comes the ‘moonlighting’ issue

Gordon Brown is hoping one of the political traps he set for the Tories in the wake of the expenses scandal will ensnare David Cameron next week, when all MPs will be required to reveal full details of their 'moonlighting' activities.

The Tories are bracing themselves for fresh revelations showing the full extent to which shadow ministers and other senior Tory MPs earn huge amounts from outside interests, often for precious little actual work.

Cameron has already suffered a rebellion over the matter when he originally suggested his frontbenchers should give up their second jobs. Shadow foreign secretary and de-facto deputy leader William Hague was said to be particularly infuriated as he was one of the Commons' top earners for his after-dinner speeches.

In the end, Cameron backed down, but the expenses affair gave Brown the opportunity to bring in new rules that will require MPs to detail their other incomes - and that will see many of them, including Tory frontbenchers, finally giving ground and abandoning some of this work.

Hague, who last year earned £230,000 from speeches, consultancies and book writing, has now started giving up his outside interests and has pledged to drop all of them by the end of the year.

Meanwhile, senior Tory Oliver Letwin, who is drawing up the party's manifesto, has confirmed he earned £60,000 a year doing eight hours work a week for the Rothschild bank and has promised to give up the work "in due course".

Similarly frontbencher David Willetts has said he will be giving up his £80,000-a-year job as consultant by the end of the year. Others, including Andrew Mitchell and Alan Duncan, have also already agreed to scale back their lucrative outside earnings.

But there are many more who have directorships and other interests and Brown hopes this will hand him further ammunition to paint the Conservatives as the party for millionaires, a theme he has already taken up in relation to Cameron's plan to cut inheritance tax.

There is the underlying issue of exactly what some of the firms employing MPs expect in return for their cash, particularly when the individual politician does little actual work for them - the implication being the aim is to exert political influence.

However, there are danger for Brown too – notably in the cases of former Labour ministers who quickly take up lucrative work with firms operating in areas linked to their previous responsibilities.

For example, former health secretary Alan Milburn is reported to be a non-executive director at Diaverum AB, a Swedish healthcare company, and has earned £25,000 as a member of Lloyd's pharmacy advisory panel.

And Ian McCartney, the former trade minister, a job which included responsibility for nuclear energy policy, is giving up a £113,000 consultancy with an American nuclear energy company. He has already announced he is standing down at the next election.

As with expenses, the 'moonlighting' revelations could end up affecting all parties - and damaging the entire political system.