Since the late 19th century, the very rich have been paying people to demand less government. The work of Herbert Spencer, for example, was sponsored by Andrew Carnegie, John D Rockefeller and Thomas Edison. Spencer believed that society changed according to evolutionary laws. Humans were evolving towards perfection, but this process was inhibited by interference from the state. By protecting people from the consequences of their own actions (or their own bad luck), it stopped the winnowing process that would otherwise result in the survival of the fittest.

Illustration by Daniel Pudles

Social security, publicly funded education, compulsory vaccination, laws enforcing safety at work all interrupted social evolution. But a self-regulated free market would swiftly ensure that those who were best adapted would survive and triumph. It's not hard to see why the millionaires loved him. They saw themselves as winners of the evolutionary race, taking their rightful place at the pinnacle of the social order. Any attempt to limit their freedoms would prevent society from achieving perfection.

Today, sponsorship by millionaires and corporations explains why free-market thinktanks outnumber and outspend the thinktanks arguing for public services and the distribution of wealth. Or so I guess. But their absence of accountability means that guesswork is all we've got. As I showed last month, only one of the rightwing thinktanks I contacted was prepared to reveal who funded it. All the others refused on the grounds that they had to respect the privacy of their donors. These organisations exert great influence in public life. But we have no means of discovering on whose behalf they do it.

Revelations about this secret funding network have now brought down a cabinet minister. Liam Fox was enmeshed in a web of corporate influence about which we still know little. The organisation he founded, Atlantic Bridge, was registered with the Charity Commission as a thinktank. Like many others, it looked more like a lobbying outfit, demanding privatisation, deregulation and tax cuts. The key question remains unanswered: who funded it?

As a result of better transparency laws in the US, we know more about Atlantic Bridge's partner organisation, the American Legislative Exchange Council. It claims, like most thinktanks, to stand for limited government and free markets. What this means in practice is lobbying against government action such as regulating tobacco and greenhouse gases. By an astonishing coincidence, it turns out to have been funded by the tobacco companies Altria and Reynolds American, by the oil giant Exxon and by the billionaire Koch brothers, who run a fossil fuel and chemicals empire they call "the biggest company you've never heard of".

Freedom is what all these groups claim to stand for. But the freedom they promote is of a particular kind. They are not campaigning for freedom from hunger or poverty. They are not demanding free access to health and education. They are not lobbying for freedom from industrial injuries, exploitation, pollution or unscrupulous banking. When these libertarians say freedom, they mean freedom from the rules that prevent their sponsors behaving as they wish: mistreating their workers, threatening public health and using the planet as their dustbin.

Like everything else about these lobbyists, the true, unacceptable meaning of the freedom they espouse is hidden behind an acceptable front. Thinktanks and lobby groups are the bane of democratic politics. They are the means by which corporations and the ultra-rich influence public life without having to reveal their hand. Their refusal to reveal who funds them, and the British state's failure to demand it, are deeply undemocratic.

Last week in the Guardian, Michael White wondered why Liam Fox did not make his friend Adam Werrity an officially sanctioned special adviser. Had he done so, Werrity's presence in his department would not have broken civil service rules, and Fox might still be in his post. But it would also have meant that Werrity's activities would have been subject to freedom of information requests, and that could have been fatal to what he was doing.

What this case highlights is the asymmetry of information in public life. The public sector is now so transparent that we have a right to read the private emails of climate scientists working for a state-sponsored university. The private sector is so opaque that we have no idea on whose behalf the people who appear every day on the BBC, using arguments that look suspiciously like corporate propaganda, are speaking.

The Labour government weakened the rules on lobbying transparency. The ministerial code published in 2007 dropped the requirement that meetings between ministers and lobbyists should be recorded. It also rebuffed MPs' demands for a register of lobbyists. You'll be surprised to hear who the villain was: Tom Watson, then a Cabinet Office minister, now a heroic campaigner for corporate accountability. He brushed aside the call for a register with the claim that "we have a pretty good system in the UK". In fact, we have no system at all: the Commons public administration committee has pointed out that "Lobbying activity in the United Kingdom is subject to no specific external regulation."

Thanks to the Fox scandal, the coalition government will now be forced to do something. But unless new legislation also applies to the thinktanks, their funders will keep using them to promote their interests without disclosure. The law should insist that all organisations which seek to influence public opinion should reveal sources of funding greater than £1,000.

The government might also take a look at charity law. It seems remarkable to me that groups such as Policy Exchange, the Institute of Economic Affairs and the Global Warming Policy Foundation have charitable status. The Charity Commission disqualified Atlantic Bridge on the grounds that "it is not permissible for a charity to promote a particular pre-determined point of view". Should this not disqualify all of them? Can you imagine the IEA deciding that private companies should get their noses out of the NHS? Can you picture Lord Lawson's Global Warming Policy Foundation announcing that climate change is an urgent threat and fossil fuel companies need stricter regulation? Is it credible that these organisations do not have "a particular pre-determined point of view"?

And shouldn't it be a basic requirement of charity law that we know who, as taxpayers, we are subsidising? How can an organisation qualify as a charity if we don't even know whose interests it is promoting? I strongly suspect that we are granting tax breaks to multimillionaires and corporations to help them change public opinion. I invite the thinktanks to prove me wrong.

Let's also demand that the BBC reform its editorial guidelines, so that no one working for a group whose purpose is public advocacy can take part in a programme unless it has published a registry of interests. Otherwise the BBC is granting free airtime to corporations without disclosing who they are or what their interest in the question might be.

So come on you free-market libertarians, let's hear your arguments against transparency and accountability. And let's hear how you reconcile them with your professed love of freedom.

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