Her Majesty's Revenue & Customs is once again demanding money with menaces. If you haven't completed your 2009/10 self-assessment tax return, says its latest press release, you must "file online or you could face a £100 penalty". If you hide your wealth or make an honest mistake, it will hit you with fines, interest payments and, ultimately, a criminal prosecution.

That's the way it goes. Or, rather, that's the way it used to go. Under the freewheeling, happy-go-lucky leadership of Dave Hartnett, the Revenue's head of tax, readers might now think about trying their luck. Suppose you have an outstanding bill of £7,250. You phone Hartnett and arrange to meet him for dinner. It is ridiculous for the Revenue to expect you to pay the full £7,250, you say. You will give him £1,250 instead. Not right away, of course. Times are hard and money is tight. The best you can manage is £800 now and the remaining £450 some time before 2015.

Alas, I regret to inform you that Dave is unlikely to cut you any slack. He may not even return your calls. You, after all, are an ordinary British taxpayer, who must pay on demand or face the consequences. If, however, you were multinational company, Hartnett would be indulgence personified. For Vodafone, HMRC reduced a potential liability not of about £7,000 to a little over £1,000 but of about £7bn to a little over £1bn and left the second-largest company on the stock market with a remarkably light tax bill.

I don't think that HMRC realises it yet, but the Vodafone scandal is as devastating for its reputation as the banking crisis was for the reputation of the financial regulators. It shows that the Revenue is prepared to have one law for the wealthy and another for the rest and undermine the moral basis of the system over which it presides.

Although tax law is usually described as fiendishly complicated, the facts of the case are straightforward. Vodafone bought the German firm Mannesmann for £112bn and channelled the loans it raised for the purchase through a subsidiary in the Luxembourg tax haven. It lent the money on to Vodafone's German operation. Revenue inspectors were clear that the billions in interest payments Vodafone had built up virtually tax-free in Luxembourg should be liable for tax in Britain.

The Court of Appeal helped when it decided that British regulations ruling out the use of tax havens for avoidance schemes did not conflict with European law, as the corporation claimed.

The gratified inspectors were pushing ahead when Hartnett intervened. In an interview with the FT, he treated his staff with enormous condescension. They were "very intelligent people", he conceded, but too "tough" and too "black and white about the law".

He must work for a different Revenue from the one the public encounters. Ours deals solely in black and white: if you owe money you have to hand it over. Hartnett's Revenue prefers shades of grey. Private Eye, which has conducted a meticulous investigation, showed how he pushed aside the staff who were pursuing Vodafone. He arranged a settlement with John Connors, Vodafone's head of tax, who, funnily enough, was a senior official at HMRC until 2007, where he worked closely with Hartnett. We are on a visit to a very small world.

Vodafone would pay a mere £800m upfront and another £450m over the next four years, they agreed, leaving an estimated £6bn of potential tax revenue in its coffers. Hartnett indicated that he will extend the same leniency to other corporations when he said he wanted a less combative approach to business.

HMRC's press office dismisses the £6bn tax write-off as an "urban myth". But the Eye's calculations on the lost income are based on publicly available accounts which detail the vast wealth of Vodafone's Luxembourg subsidiary. They add in the lost income that comes from the Revenue allowing Vodafone to continue with other tax-reducing wheezes. The Revenue, by contrast, offers questioners nothing beyond bluster and unsubstantiated assertion.

Hartnett strikes me as another victim of "regulatory capture". Like Gordon Brown and Mervyn King in the bubble, he seems in awe of the movers and shakers of big business and far too pleased to spend time in their exalted company. A freedom of information search found, for instance, that he had gone to a Monte Carlo hotel to dine with the tax specialists at KPMG, whose day job is thinking up ever-more ingenious ways for their wealthy clients to dodge tax.

The effect on British attitudes of HMRC's sweetheart deals for the rich is hard to gauge but it could be severe. We have a Protestant north European culture. We are not like the Italians and Greeks who see nothing immoral in fiddling the system. On the whole, we believe that we must pay tax and obey the law whether we like it or not. Cheating the taxman is still a dirty secret for a few and not something they boast about, even when they are among friends. Most of us submit to HMRC's demands with grumbling reluctance.

But national cultures change. Fifty years ago, the British still thought the Germans were dangerous extremists. Now, Germans practise the stolid virtues of thrift and moderation we once thought our own, while the British, who once abhorred debt, have become a nation of maniacal spendthrifts.

If the double standards of the tax authorities sink into the public consciousness, HMRC risks shifting cultural attitudes and making tax evasion a respectable response to a morally bankrupt system. It will be ill-equipped to deal with recalcitrant taxpayers should they appear in large numbers. HMRC's computer systems are unusually useless even by Whitehall standards. Meanwhile, Labour has cut 30,000 jobs and the coalition is cutting 12,000 more. HMRC needs the co-operation of the public more than it realises.

All articles about tax and the wealthy end up quoting Leona Helmsley. She seemed a grotesque figure in the 1980s, even though the amounts she dodged were pin money by today's standards. In 1989, a New York court jailed the Manhattan "hotel queen" for tax evasion after her housekeeper testified that Helmsley had said to her: "You must pay a lot of taxes. We don't pay taxes. Only the little people pay taxes." Once, "only the little people pay taxes" was the slogan of convicted fraudsters. Now, it sounds uncomfortably close to becoming the mission statement of Her Majesty's Revenue & Customs.