Police attitudes have progressed by light years since the bad days of the 1980s and highly publicised miscarriages of justice. So runs the prevailing wisdom. But the recent police operation at the G20 summit and instinctive reaction of some officers to doctor the facts indicates that a deep-seated corruption in policing still prevails.

It reared its head after the killing of Jean Charles de Menezes and stems from a self-righteous arrogance within police forces and a belief, held widely by many officers, that they are above the law. It leads to attitudes that have led to manipulation of evidence and wrongful convictions: victims include Colin Stagg and Barry George.

The history of the criminal courts is littered with false convictions caused by overzealous and dishonest policing. More than 30 years ago police procedures made it easy to concoct and manipulate. These were epitomised by the ubiquitous ?verbal?.

Until 1984 interviews with suspects were conducted in the absence of a defence solicitor and without notes being taken. Officers would get together later and write up an account of the conversation, invariably containing a confession.

Many criminals claimed falsely that they had been ?verballed up? but there was a large number of cases in which police officers concocted confessions. Verballing was endemic in every force, including Lancashire, where the new Metropolitan Police Commissioner cut his investigative teeth ? although I do not suggest that he was involved.

The practice was obvious to those of us who defended in the criminal courts. A 25-minute interview would, when reduced to writing, consist of a couple of pages of handwritten notes. When it was suggested to police officers that a great deal of conversation had been omitted, they would claim that they had faithfully recorded all questions and answers. Many judges seemed blind to police dishonesty and, although juries tended to be more sceptical, there were numerous cases that resulted in convictions based on lies.

Judicial myopia reached the very top. In the appeal of the Birmingham Six, Lord Denning said: ?If the six men win, it will mean that the police are guilty of perjury, that they are guilty of violence and threats, that the confessions were invented and improperly admitted in evidence and the convictions were erroneous . . . this is such an appalling vista that every sensible person in the land would say that it cannot be right that these actions should go any further.?

Eleven years later it was accepted that the ?appalling vista? was no less than the truth.

The introduction of contemporaneous notes meant that there were fewer confessions, which was inconvenient for the police. It had been all too easy to threaten or mistreat a defendant into a confession or, if that failed, to invent one. Now officers had to write down the conversation as it happened. Ever resourceful, however, new methods were found to overcome the difficulty.

Notes were doctored with omissions or additions; in 1989 it was discovered that interview notes with Patrick Armstrong, one of the Guildford Four, had been heavily edited and rewritten before trial. The four were released after spending 15 years in jail.

In 1976 Stefan Kiszko was convicted of murdering an 11-year-old girl. He was interviewed without a solicitor. As a result of threats, which were not alluded to in the notes or evidence of the police officers, he confessed and spent 16 years in jail before his innocence was proved.

In 1977 Reg Dudley and Bob Maynard were convicted of murder. They spent 20 years in jail but in 2002 it was established that their confession notes had been fabricated.

The introduction of tape-recording prevented the doctoring of interviews. But the culture of falsity, of deciding on guilt and fitting evidence to the decision, had become ingrained. Its tentacles spread into other spheres of police investigation.

In recent years, I can attest that officers from Lancashire, Liverpool and Northumbria have used a variety of ingenious methods in attempting to secure the convictions of innocent men. These cases resulted in acquittals: they were ?carriages of justice? that could so easily have become miscarriages to rank with Kiszko, the Birmingham Six and many others.

The police have a difficult job and many officers are fair. I pay tribute to the thousands who would not dream of lying or of manipulating evidence, who are capable of being objective, who are not blinkered in their approach to an investigation. Nor do I suggest that any police officer sets out to secure the conviction of people he or she believes to be innocent. The danger to liberty and to the rule of law lies in the tendency of some to decide on guilt first and then look for evidence in support.

It is this tendency with its deep-rooted origins in the history of the police service that leads to injustice. It is this tendency that can and does lead to the conviction of the innocent. And it stems from the same malaise that causes the overreaction and brutality we saw at the G20.