The auction of spending promises is over, and the next government will now have to find ways of paying the bills.

With higher taxes a taboo for any campaigning party, a prominent theme for all of them has been the populist pledge to crack down on tax dodging. The Conservatives promised to raise a further £5billion, Labour £7.5bn, the LibDems £10bn, and the Greens a mind-boggling £30bn. But how can they do it?

James Browne, senior research economist at the IFS, has criticised all three main parties for using figures "plucked out of thin make sure all their sums add up".

Since 2010, the tax system has become even more complex and open to gaming, while HM Revenue & Customs has been criticised for failing to use its powers.

An investigation by the Public Accounts Committee, chaired by Labour MP Margaret Hodge, concluded in March that a significant obstacle was the "too cosy"relationship between HMRC and the big accountancy firms which promoted tax avoidance schemes to wealthy clients and companies.

Those big firms, of course, lure many of HMRC's brightest brains to become gamekeepers turned poachers.

MPs said the taxman was not doing enough to pursue serious evasion, and criticised the"tax avoidance industry" of accountants, advisers and lawyers, "making lucrative business out of designing and selling ways for clients to avoid tax".

Mrs Hodge said: "HMRC needs to change the perception that it is far too tolerant of these companies and individuals - in contrast to its treatment of small businesses and the majority of the public who pay taxes through PAYE."

The Labour MP had previously highlighted how political parties get "inappropriate" support from the big accountancy firms including PwC, said in the report to be the biggest promoter of avoidance schemes, which has provided seconded advisers to both Tory and Labour shadow ministers.

But the committee also warned that only a simplifying of the UK's horrendously long and complex tax code would begin to root out evasion. Tax reliefs have increased during the past five years by over 100 to 1140.

The report concluded: "Tax reliefs add to the complexity of the system and may be exploited as a way of avoiding tax. HMRC does not effectively monitor changes in the cost of tax reliefs so is slow in identifying instances where a relief is being exploited for a purpose Parliament did not intend."

Mrs Hodge said: "HMRC could collect more of the tax that is due if it had more resources devoted to this work and it should be assertive about making this case to HM Treasury and Parliament."

Avaaz, an online campaign group, is this month threatening to ask for judicial review of HMRC's policy of offering amnesties to wealthy bank customers such as those known to have secreted assets in Swiss banks.

Despite being given a leaked database of 3600 HSBC customers in Geneva five years ago, the tax agency has brought only one prosecution.

Avaaz has asked HMRC to explain why it has deployed the Liechtenstein Disclosure Facility (LDF), which allows tax dodgers to pay the tax avoided plus a 10 per cent fine, and avoid prosecution, rather than use powers allowing fines of up to three times the sum due.

The LDF involves individuals moving their money to banks in Liechtenstein, which certify them and disclose them to HMRC, and also allows them to escape any pre-1999 assessment.

The group claims HMRC has foregone large sums in penalties and lost the opportunity to bring criminal prosecutions that would act as a deterrent, because in the HSBC case the evaders' identities were already known so an amnesty for unknown dodgers was unnecessary.

The public accounts committee also questioned whether deals such as the LDF were too generous to those whose wealth can be stashed away in tax havens.

Mrs Hodge, ironically, has admitted that she herself has benefited from the LDF, when shares were transferred to her in 2011 following the winding-up of an offshore family trust which held a £7.7m stake in the company run by her brother.

HMRC has said 1100 HSBC customers settled using the LDF, that its amnesties have been highly efficient in avoiding costly legal action, and that since 2010 its clampdowns have raised £100bn in extra revenue.

It says: "We have shut down marketed avoidance schemes, closed loopholes, secured tough new enforcement powers, and opened up international information exchanges so rich evaders will have no safe havens where they can hide their money."

Labour promised in its manifesto that both the Chancellor and the HMRC chief executive would in future present an annual report to Parliament, and give evidence to the Treasury select committee, on the government's progress in tackling tax avoidance and evasion.

But Paul Johnson, director of the IFS, said tax avoidance arose because "we charge tax at different rates on capital gains ... we have a national insurance system which provides a big incentive to take money other than in earned income and we tax corporate income in all sorts of different ways....that's why we are continually putting sticking plasters on."