He tells BBC News Online the case of the two spies unmasked at the weekend shows how MI5 uses the excuse of security to cover up its blunders.

With the Melita Norwood affair, MI5 is once again making the government look stupid.

As I have said time and time again, secrecy in the UK is used against the public interest rather than for it. Commentators have accused me of being a traitor for adopting this position. It is good now to be vindicated.

Two years ago, I offered evidence to the government and the Intelligence and Security Committee. Both effectively refused to take my evidence.

Of course, if they had taken it then, they would have known that MI5 had failed to deal properly with two other traitors (in addition to those revealed over the weekend).

Both of these people sold defence secrets to the Soviet Union during the Cold War. Neither has been prosecuted. In one case, the decision not to prosecute came about as a result of MI5's failure to warn the suspect of their rights. I do not know why MI5 did not proceed in the other.

Excessive secrecy

In all of this, there is one common factor. Secrecy has been used to work against the public interest by covering up MI5's blunders.

Once again, our excessive secrecy legislation has been used to hide the embarrassment of MI5 rather than to protect the security of the state. It is exactly the same kind of thinking that has characterised the government's and MI5's attitude to my public-spirited whistleblowing.

Shayler has taken to the Web to argue his case

A judge and a Special Branch officer have concluded that my disclosures to the Mail on Sunday did not damage national security. But they did of course embarrass the intelligence services by focusing on their incompetence.

Secrecy is so ingrained in MI5's culture that it will not even debate with outsiders why it needs secrecy legislation, which is a disgrace by the standards of a liberal democracy to protect it from criticism.

Ministers kept in the dark

While working for MI5, I was routinely told not to include too much detail in reports disseminated to government. The reason for this was not to do with the security of MI5's sources, but simply to avoid hassle for MI5 in future.

If ministers had accurate and detailed information, they would have been able to hold MI5 to account over its progress in investigations. Without it, ministers had to accept MI5's own assessment of its performance in specific operations and in general.

To put it bluntly, ministers responsible for MI5 were left feeling grateful that MI5 bothered to confide in them at all. I have a very good example of this.

In March 1995, I prepared a report on terrorism which was designed to bring Whitehall up to date with key developments at the time. When I left the Service in late 1996, it had still not gone out to MI5's masters in government, even though it concerned major new information that was vital to the government in formulating policy towards a particular country.

It is a great journalistic myth that MI5 will hand a 'file' or 'dossier' to government. Nothing could be further from the truth. MI5 usually disseminates brief reports to Whitehall which contain mostly assessment, but with little detail of the actual intelligence or the source of it.

Government 'colluded' with MI5

Of course, when I first went on the record with my very reasonable criticisms of MI5, the government had two options. It could have changed the culture of secrecy in Britain by having an open inquiry into what I had to say and what MI5 had to say about my criticisms.

This would of course been in line with the principles of natural justice and Labour's election commitments. It would also have increased parliamentary and public confidence in the secret state in the long term.

Instead, the government chose a second option. It chose even to refuse to take possession of my intelligence, even though this highlighted an illegal operation carried out by MI6. In other words, ministers decided to collude with the intelligence services in a cover-up.

Greater openness needed

As MI5 becomes more and more involved in routine crime, it has to open itself up to proper scrutiny. I cannot imagine, say, in a murder enquiry, the police expecting to get away with not even interviewing a suspect, no matter how long ago that murder took place. But that appears to be precisely what happened in the Melita Norwood case.

Similarly, I cannot imagine the media and parliament being happy with Sir Paul Condon, a Crown servant like intelligence chief Stephen Lander, giving off-the-record briefings to selected journalists about, say, the Stephen Lawrence murder.

But that is what happened in my case. Mr Lander has not said one word on the public record about my disclosures. He should not be allowed to get away with his silence again.