On Tuesday, the Financial Times splashed on a remarkable story about tax. The paper's Vanessa Houlder had uncovered a National Audit Office study showing nearly one-third of the biggest 700 businesses in the UK paid not one penny in corporation tax in 2005/06. Tax experts queued up to tell the paper how taken aback they were by the news. Rather than surprise, one might have thought they would be trump-eting it as the ultimate vindication of the quality of the tax planning services people like them provide.

But no. Such a stark revelation, coming hard on the heels of tales of private equity tyros paying less tax on their zillions than their cleaning ladies, and the global super-rich making London their home, while clinging to non-domiciled status when it comes to completing their UK tax returns, might just trigger a public backlash. So the tax experts focused on which businesses are still paying some tax on their profits.

Small- and medium-sized businesses paid 46% of the £44.3bn raised from UK corporation tax last year. But the 54% which came from their larger counterparts is very unevenly distributed. Around 220 big companies, as we've seen, paid nothing. According to the NAO study, another 30% of the largest 700 paid no more than £10m each. Just 50 companies in that group, a mere 7% of the total, contributed 67% of all the corporation tax raised from big corporates in 2005/06.

That's not quite 7/84 proportions - the old ratio of how unequally land ownership was spread across Scotland which inspired a remarkable theatre company. But it comes pretty close. And since the minority of really big corporate taxpayers is clustered in the banking, oil and gas, and insurance sectors, an important source of government revenue looks very exposed to any downturn in the financial sector or a sustained fall in the oil price.

But there's a bigger question in all this. How do so many companies in the UK get away with paying absolutely no tax at all on profits?

Some on these islands, including the new government at Holyrood, argue that the prerequisite of greater economic dynamism is lower taxes on business. But if so many big companies can already reduce the tax bill on their profits to zero, why would cuts in the nominal rate at which that tax is charged hold even a passing attraction?

It's not that these businesses are doing anything illegal. They are simply exploiting Britain's hugely complex tax codes, with their myriad write-offs and exemptions, to the full, to minimise their UK exposure. Like all those Russian oligarchs, Saudi princes and Greek shipping magnates who have flocked to London to take advantage of our remarkably generous approach to where they choose to call home, many multinational businesses clearly find nominally high-tax Britain just as congenial a place to do business as any offshore tax haven. The non-domiciled community, perhaps as many as 60,000 of them, are free to live here for as much as they like while calling somewhere else their real home.

It's a throwback from the days of empire, but it means they can shelter all their assets and earnings outside Britain from UK tax. No other major nation on the face of the planet, not even the US, feels it necessary to be that generous.

The current rules on corporation tax allow highly-geared multi-nationals to base themselves in the UK while offsetting the cost of all their borrowing - wherever around the world the resultant investment is directed - against their UK corporation tax bill.

That's one major reason why 220 of our biggest corporates paid nothing at all in 2005/06. It's not the only one, of course.

Companies can also write off tax liabilities against such things as capital investments made here, payments into pension funds and past years' accumulated losses. But it's the ability of multinationals in sectors like construction, business services, automotive, real estate, and alcohol and tobacco to use their gearing to minimise their UK tax liability that makes the decisive difference.

The Treasury has been looking at ways of restricting such relief. But it has tended to shy away from any moves that might brand Britain as a less open economy or threaten some of the jobs that such openess clearly generates here.

The UK's approach to taxing company profits may be, as the FT asserted, "something of a mess". But there appears to be little appetite in government to publicise the kind of anomalies the present system throws up.

Tuesday's story, which surprised even the tax professionals, was not the result of the NAO routinely publicising one of its latest studies. The paper, which first appeared in July, was more concerned with how HM Revenue & Customs "delivers services to big business" than with highlighting how many big companies were failing to pay any corporation tax. The executive summary said nothing about one-third of big corporates paying no corporation tax at all, nothing about how unevenly revenues were spread.

You had to dig down, to the analysis and charts from page 10 onwards, to get to the surprising bits. HM Revenue & Customs is charged with improving its compliance work, which currently boosts corporation tax revenues from big business by between £1.9bn and £2.7bn a year. But everyone involved seems very reluctant to highlight how effective tax avoidance can be in Britain if your are a highly-geared multinational.

The NAO study was simply following up on last November's Treasury-inspired review of links between the Revenue and large businesses, carried out by Sir David Varney for the then-Chancellor Gordon Brown. In it, Varney talked of "government and business having a common goal of maintaining and enhancing the attractiveness of the UK as a place to do business in and to do business from".

There was plenty of talk about greater clarity, certainty and delivery in how that relationship evolves. But nothing about the anomalies buried in the later NAO study. And nothing about one crucial dimension of any tax system: fairness.

Defenders of the way UK corporation tax currently operates insist all companies, even those which don't pay it, contribute in other ways to the national exchequer, through employers' national insurance contributions, unrecoverable VAT and so on. But ignoring the manifest inequalities in the way corporation tax currently operates, across sectors and between the highly-geared multinationals and smaller domestic UK businesses, is not sustainable long term.

As Prime Minister, Gordon Brown is not afraid to tell public-sector workers they must moderate their pay claims or risk fuelling inflation. He should also have the courage to confront those in business who are, at present, taking our tax system for a ride.