David Shayler worked for MI5 and is alleged to have disclosed a series of concerns about his employers -- for example, that they kept files on Jack Straw and Peter Mandelson -- in the Mail on Sunday almost exactly three years ago.The government obtained an injunction to stop the second article in the newspaper's series.Mr Shayler was arrested in Paris in August a year later, just as the 'Qadhafi plot' story was going to break in the media.

The Qadhafi plot concerned allegations that MI6 officers had encouraged and paid dissident groups in Libya to kill the country's president, and that civilians died as a result.

Mr Shayler returned from France last week, was charged under section 1 of the Official Secrets Act (OSA), and will put his case to a jury.There is an important tradition of whistle-blowing in this country.

Cathy Massiter blew the whistle on MI5 spying activities against the civil liberties group Liberty, CND and others.

As a result of Liberty's complaint, using her evidence to the European Commission on Human Rights, the government brought forward the Security Services Act 1989 which imposed the first elements of legal control over the services.

But in the absence of an adequate system of accountability for the security services, we rely on whistle-blowers to expose malpractice.There is now some protection for whistleblowers, but it does not apply to MI5 officers.

Section 1 of the OSA provides no defence where a disclosure was made in the public interest or caused no damage to national security.

This is one reason for Liberty's involvement in David Shayler's case.

Liberty wants to take advantage of the fact that the Human Rights Act is likely to change the way the OSA is interpreted.

Section 3 of the Human Rights Act imposes a duty on the courts -- 'so far as it is possible to do so' -- to interpret other laws so that they comply with the rights in the European Convention on Human Rights.

The key right in this instance is article 10, freedom of expression.

(In the late 1980s Tony Blair, Jack Straw and many other current cabinet members voted in parliament against the bill that created the OSA because it did not include a public interest defence.)The OSA, in Lor d Irvine's words in 1996, 'was no liberalising measure', 'provides no positive rights to information and no freedom of information' and 'fails to recognise the public interest to know of abuses by government of its powers'.This right in article 10 is not, of course, absolute and restrictions on it can be made for the purposes of national security or for the prevention of disorder or crime.

But any interference with article 10 can only be justified if it is aimed at one of these purposes, the interference is regulated by the law and it is a 'proportionate' response.This means the OSA cannot create criminal offences which restrict freedom of expression unless this is a proportionate response to the damage caused or the threat to national security.

The courts must weigh up the damage caused against the public interest in disclosure.

Offences created by section 1 of the OSA do not pass this test because of the absence of any test of damage.

Secondly, the public interest in disclosure must be a factor that the court -- and the police and prosecutors -- takes into account when deciding whether an offence has been committed (or what the sentence should be).Section 3 of the Human Rights Act, says the Lord Chancellor, will allow the courts to read words into legislation to ensure compatibility with the Convention.The OSA will need to be interpreted in a different way after 2 October and in a way that gives more respect to freedom of expression and the public interest in disclosure.

David Shayler's case will ensure this happens quickly.