'I would love to see tax reductions," David Cameron told the Sunday Telegraph at the weekend, "but when you're borrowing 11% of your GDP, it's not possible to make significant net tax cuts. It just isn't." Oh no? Then how come he's planning the biggest and crudest corporate tax cut in living memory?

If you've heard nothing of it, you're in good company. The obscure adjustments the government is planning to the tax acts of 1988 and 2009 have been missed by almost everyone – and are, anyway, almost impossible to understand without expert help. But as soon as you grasp the implications, you realise that a kind of corporate coup d'etat is taking place.

Like the dismantling of the NHS and the sale of public forests, no one voted for this measure, as it wasn't in the manifestos. While Cameron insists that he occupies the centre ground of British politics, that he shares our burdens and feels our pain, he has quietly been plotting with banks and businesses to engineer the greatest transfer of wealth from the poor and middle to the ultra-rich that this country has seen in a century. The latest heist has been explained to me by the former tax inspector, now a Private Eye journalist, Richard Brooks and current senior tax staff who can't be named. Here's how it works.

At the moment tax law ensures that companies based here, with branches in other countries, don't get taxed twice on the same money. They have to pay only the difference between our rate and that of the other country. If, for example, Dirty Oil plc pays 10% corporation tax on its profits in Oblivia, then shifts the money over here, it should pay a further 18% in the UK, to match our rate of 28%. But under the new proposals, companies will pay nothing at all in this country on money made by their foreign branches.

Foreign means anywhere. If these proposals go ahead, the UK will be only the second country in the world to allow money that has passed through tax havens to remain untaxed when it gets here. The other is Switzerland. The exemption applies solely to "large and medium companies": it is not available for smaller firms. The government says it expects "large financial services companies to make the greatest use of the exemption regime". The main beneficiaries, in other words, will be the banks.

But that's not the end of it. While big business will be exempt from tax on its foreign branch earnings, it will, amazingly, still be able to claim the expense of funding its foreign branches against tax it pays in the UK. No other country does this. The new measures will, as we already know, accompany a rapid reduction in the official rate of corporation tax: from 28% to 24% by 2014. This, a Treasury minister has boasted, will be the lowest rate "of any major western economy". By the time this government is done, we'll be lucky if the banks and corporations pay anything at all. In the Sunday Telegraph, David Cameron said: "What I want is tax revenue from the banks into the exchequer, so we can help rebuild this economy." He's doing just the opposite.

These measures will drain not only wealth but also jobs from the UK. The new legislation will create a powerful incentive to shift business out of this country and into nations with lower corporate tax rates. Any UK business that doesn't outsource its staff or funnel its earnings through a tax haven will find itself with an extra competitive disadvantage. The new rules also threaten to degrade the tax base everywhere, as companies with headquarters in other countries will demand similar measures from their own governments.

So how did this happen? You don't have to look far to find out. Almost all the members of the seven committees the government set up "to provide strategic oversight of the development of corporate tax policy" are corporate executives. Among them are representatives of Vodafone, Tesco, BP, British American Tobacco and several of the major banks: HSBC, Santander, Standard Chartered, Citigroup, Schroders, RBS and Barclays.

I used to think of such processes as regulatory capture: government agencies being taken over by the companies they were supposed to restrain. But I've just read Nicholas Shaxson's Treasure Islands – perhaps the most important book published in the UK so far this year – and now I'm not so sure. Shaxson shows how the world's tax havens have not, as the OECD claims, been eliminated, but legitimised; how the City of London is itself a giant tax haven, which passes much of its business through its subsidiary havens in British dependencies, overseas territories and former colonies; how its operations mesh with and are often indistinguishable from the laundering of the proceeds of crime; and how the Corporation of the City of London in effect dictates to the government, while remaining exempt from democratic control. If Hosni Mubarak has passed his alleged $70bn through British banks, the Egyptians won't see a piastre of it.

Reading Treasure Islands, I have realised that injustice of the kind described in this column is no perversion of the system; it is the system. Tony Blair came to power after assuring the City of his benign intentions. He then deregulated it and cut its taxes. Cameron didn't have to assure it of anything: his party exists to turn its demands into public policy. Our ministers are not public servants. They work for the people who fund their parties, run the banks and own the newspapers, shielding them from their obligations to society, insulating them from democratic challenge.

Our political system protects and enriches a fantastically wealthy elite, much of whose money is, as a result of their interesting tax and transfer arrangements, in effect stolen from poorer countries, and poorer citizens of their own countries. Ours is a semi-criminal money-laundering economy, legitimised by the pomp of the lord mayor's show and multiple layers of defence in government. Politically irrelevant, economically invisible, the rest of us inhabit the margins of the system. Governments ensure that we are thrown enough scraps to keep us quiet, while the ultra-rich get on with the serious business of looting the global economy and crushing attempts to hold them to account.

And this government? It has learned the lesson that Thatcher never grasped. If you want to turn this country into another Mexico, where the ruling elite wallows in unimaginable, state-facilitated wealth while the rest can go to hell, you don't declare war on society, you don't lambast single mothers or refuse to apologise for Bloody Sunday. You assuage, reassure, conciliate, emote. Then you shaft us.

• A fully referenced version of this article can be found on George Monbiot's website