First, the UK Bill of Rights Commission: has anyone seen it? Are we quite sure it exists in corporeal form? Do we know where its members meet? How come people are forming the impression that the UK Bill of Rights Commission is doing rather less for the public good than a home counties bridge party?

These questions haunt me because Kenneth Clarke, the secretary of state for justice, the man who set up the commission last spring to investigate a new bill of rights – no doubt with half an eye on the 800th anniversary of Magna Carta in 2015 – is also responsible for the justice and security green paper, which threatens to deprive us of one of the vital traditions of common law, guaranteed by Magna Carta. You could dignify this with the word irony, but it is just the usual story of politics and hypocrisy. While pacifying those who worry about liberty with a footling commission, composed largely of lawyers from left and right, who cancel each other out, Clarke proposes a vast extension of secrecy in the civil courts and inquests, which will suppress evidence of corruption and negligence in high places, as well as reduce access to justice and the public's right to know.

Sometimes, I wonder if Clarke, who is a QC, read the paper before it was published last autumn, because it is arguably more menacing to our legal traditions than anything dreamed up by his predecessor, Jack Straw. But he must have read it. Indeed, the high commands of the major parties have all read and apparently support the proposals that, in the case of the Liberal Democrats, amount to a historic betrayal of the principles of open justice.

Apart from the Tory MP David Davis, there is barely a whisper of opposition to the shoddy, self-serving and manifestly illiberal measures, which Clarke claims will improve executive accountability and provide "a court system equipped to deal with sensitive material and intelligence services that are able to get on with their job".

There you have the lies summarised. Now let's look at what the proposals will actually mean.

The most important point is that Clarke will provide a magic cloak of protection for any minister or government agency that wishes to cover up a wrong, most significantly for members of the intelligence services. Under his law, evidence that British officials were involved in the rendition and torture of British resident Binyam Mohammed could have been suppressed. Evidence that emerged last autumn that British spies arranged the rendition of two Libyan opposition figures to Gaddafi's people for torture may not, if Clarke gets his way, be heard in an open British court, because the minister will be able to declare the material "sensitive" or "against the public interest".

Those are vague concepts. Anything can be thus defined: the death of someone in police custody, the contamination resulting from a nuclear accident, the details of a government contract, or security lapses in a government laboratory, to say little of the matter of gentlemen with Muslim names being conveyed to some fetid, bloodstained cellar. Executive accountability will be reduced, not improved.

Once Clarke's apparatus is in place, it is likely to encourage a sense of immunity in officials, possibly the idea that they are above the law. The result will be a worse-run country, where the state grows incrementally more heedless and irresponsible – exactly the argument I made against, among other things, Jack Straw's proposal for secret inquests.

Over the last six years, I have stressed the dangers of the tendency of illiberal practices to spread through the system, after being introduced to deal with one discrete issue. Following controversy over the deportation of terror suspects, Labour established the Special Immigration Appeals Commission and introduced the "closed material procedure", which allowed evidence to be withheld from the individual and his legal team. Instead, it was revealed to a special advocate who supposedly protected the individual's interests, yet wasn't allowed any contact with him or his lawyers.

This shameful arrangement spread quickly. In numerous other contexts, the state used the "sensitive" label to make sure that evidence was not disclosed to the other party. Clarke's innovations will mean that closed material procedures and special advocates will become much more common in civil courts and inquests, denying the public knowledge of the misbehaviour and mistakes of officials – the people we pay for with our taxes. Our servants! It is retrograde and condescending in the extreme, but, worse still, it breaks the promise in Magna Carta, which says: "To no man will we sell, delay or deny justice."

Dinah Rose QC summarised the obvious advantage to ministers in her Atkin Memorial lecture last year. The legislation would, she said, "permit courts to try common law claims for damages using a closed material procedure, whenever a government minister, who is, of course, likely to be party to the action, decides that disclosure of particular material would be damaging to national security".

We are following America, where the state secrets privilege results in the exclusion of evidence from the proceedings simply on the basis of affidavits delivered to a court by the government, and this is going to make life very difficult for serious journalism in Britain. A response to the green paper from Guardian News and Media, owners of this newspaper, says Clarke's proposal would have a serious impact on the judicial process, court reporting and public interest journalism. Closed hearings, secret evidence and secret pleadings and judgments will result in the indefinite removal of information from the public domain.

The green paper is designed almost shamelessly to prevent information concerning such things as British involvement in Binyam Mohammed's torture reaching us. That motive tells you all you need to know about the rottenness of the proposal and why we should follow the example of the many special advocates who have voiced their opposition.

Clarke's fall-back position will probably be to guarantee the intelligence services immunity from scrutiny in open court, but given the absolute lack of opposition in Parliament he may get everything he wants, in which case we will all be worse off.

Which brings me to the UK Bill of Rights Commission and my conviction that if we are to have a new bill of rights, it is the public who must wrest this process from the lawyers, academics and former civil servants, working away to boost the credentials of politicians at the next election. Just as capitalism needs reform, so do the mechanisms that guarantee freedom, scrutiny and accountability. The English gave Magna Carta to the world 800 years ago; now we need to update it properly.