There is no one particular reason why David decided to go on the record. During his time in MI5, he became gradually disillusioned with what he saw going on around him. On induction courses, MI5 spends a great deal of time convincing new recruits that the Service is not as it appears in the media. But David was disturbed by many of the aspects of the work in his posting to F2, the counter-subversion section. As the section was in the process of being closed down, David was prepared to believe that the large scale monitoring of political activists belonged to another era.

However, in T2, the counter-IRA section, he became concerned at MI5's ability to sweep problems under the carpet and continue to make the same mistakes rather than learn from them. As a result, attacks like Bishopsgate and two other large-scale attacks went ahead, endangering life, when they could easily have been prevented. In the case of the latter two attacks, a senior MI5 officer lied in a report disseminated to the Queen and government to cover up MI5's shortcomings. When David tackled the officer about this, he was told to keep quiet. David is unable at present to reveal any further details of these incidents because of the injunction.

After that, David saw other examples of MI5's failure to tackle problems during his posting to G9, which dealt with Middle Eastern terrorism. As many of you will be aware, an officer failed to react to intelligence warning of an attack on the Israeli Embassy. When the attack happened, the officer then hid it in another officer's cupboard to deflect blame.

David was also concerned about MI5's investigation into Victoria Brittain, then the deputy foreign editor of the Guardian, which included a bugging operation and a planned "covert entry" (or break-in, as it is more usually known). MI5 senior officers were so excited by the prospect of an old-fashioned investigation into what they saw as a left wing troublemaker that they forgot to check publicly available information about Brittain and her contacts, which would have rendered an expensive and intrusive investigation unnecessary.

These are the specific examples of malpractice which David felt had to be addressed. But at the same time, there were many other general factors which disillusioned David, such as:

• MI5's inability to properly manage its resources, including its new intake of officers, and to train its staff properly;
• MI5's inability to resolve problems or to make itself more efficient at a time when its resources had to go further;
• MI5's obsession with bureaucracy. It did not protect civil liberties. On occasions, it allowed terrorists to escape and put the lives of the general public at risk. David has further information which has never been revealed to the public because of the injunction;
• MI5's arrogance and its unwillingness to adapt to the modern, post-Cold War world;
• MI5's Stalinist attitude to new ideas or differences of opinion which saw plodding time servers get on while record numbers of capable officers left, disillusioned.

David raised many of these issues with his management and on one occasion was even prevented from taking his concerns to the deputy director general, when he thought this was the best route to airing his concerns. When the MI6-backed Qadhafi Plot led to the murder of innocent civilians, he decided that he did not want to be part of a system which got involved in and condoned murder.

Once he had left the Service, David continued to meet former colleagues. The vast majority had similar views on MI5's shortcomings. Disillusioned with the Service, they were actively looking for alternative employment and were angry at their treatment at the hands of MI5 management. As a result, David felt that their disillusionment and the reasons for it should be brought to a wider audience.

It would have been very easy for David to forget what he saw in MI5 and then go on to lead a quiet life. However, that raises interesting moral questions about the duty of the individual in relation to the public interest. If you know that lives have been lost or civil liberties unnecessarily invaded as a result of inefficiencies and abuses of state power and you do not act, then you support the status quo.

In other words, if human beings continued to be threatened or murdered as a result of these shortcomings, you are complicit in this activity by not bringing these deficiencies to the attention of those in charge of the intelligence services. If David had withheld details of a failed terrorist assassination attempt against Tony Blair which resulted in the deaths of innocent British civilians, he would quite rightly have been criticised (and indeed prosecuted under the Prevention of Terrorism Act for failing to disclose information known about terrorist activity). Just because the victims of the Qadhafi Plot were Libyan does not mitigate the fact they were murdered in a plot funded by the British taxpayer.

There is one reason why David decided to go on the record which seems to have been forgotten by many in parliament and the media. We live in a free society. It should our democratic right to criticise the government or the state without fear of persecution. It is the censors who should be justifying their position, not David justifying his position for criticising a highly secretive and largely unaccountable department of state. As independent observers have noted, David's disclosures did not damage national security (see link) so we are still waiting for the censors to justify their position.

In 1989, when the Conservative government tried to reform the OSA, Tony Blair, Jack Straw and John Morris, the Attorney General, voted in favour of a public interest defence. At the time, they argued that wrongdoing and abuses of power in the intelligence agencies would not be brought to the attention of the public without whistleblowers. The Labour government was also elected on a platform of freedom of information. David therefore believed that the new Labour government would take a more objective view of his disclosures than a Conservative administration. He was wrong. When in prison, he also thought that the consciences of Labour cabinet ministers would be so deeply troubled when they learnt of the Qadhafi Plot that they would have to order an immediate enquiry and allow him back to the UK to give evidence against MI6. Again, he was wrong.

Given that David strongly felt the need to make his criticisms of the intelligence community, some have asked why he didn’t take his disclosures to the Home Office or another appropriate body rather than the media. There are two very good reasons for this:

• disclosing intelligence to even the Home Office would have been a breach of the draconian Official Secrets Act. David could only legally have disclosed his information to MI5;
• MI5 had not acted on any of David's concerns while he was there. He could hardly have expected it to pass on his disclosures to the home secretary without interference as they reflected very badly on MI5, especially given its attempts to convince its political masters that it had modernised in the early 1990s.

This suspicion has been borne out by subsequent events. Since David has gone on the record, two government bodies have refused to receive his evidence (see link).