Like the emperor, Enoch Powell had seen the Tiber, but mercifully the Thames would never foam with much blood. The Wolverhampton MP's apocalyptic vision of a Britain in which Britons, "swamped" by immigrants, would come to feel alien in their own land, was never realised.

Edward Heath sacked Powell from his shadow cabinet for a perceived pitch to racist instinct which would dog him beyond the grave. Sir Edward, like many of his political generation, has since found vindication in the growing tolerance of diversity, as Britain has moved to what is routinely now described as a truly multiracial society.

Certainly Britain has come a long way from the days when blacks and Asians arrived in what they in many ways considered their homeland, only to find clubs and rooming houses adorned with two simple words of brutal rejection: "No blacks".

As the country awaits Sir William Macpherson's report on the murder of Stephen Lawrence, the assumptions so readily made about the journey and progress made are necessarily revisited. Norwell Roberts would acknowledge that much has changed, in society in general and in the Metropolitan Police in particular, over the last three decades, but it is sobering to reflect that it is just 32 years since he emerged as London's first black police officer.

It is unnerving also, to say the least, to consider the abuse to which he was subject in the 1960s and then make the connection to the attitudes revealed during the Macpherson inquiry into the murder of a black teenager just six years ago.

Norwell Roberts graduated from the police training college on the same day as Sir Paul Condon, the Metropolitan Commissioner whose job now hangs in the balance in face of Macpherson's conclusion that his force is afflicted by "a pernicious and institutionalised racism". When he retired two years ago, Mr Roberts said that, had he known what to expect, he would never have joined.

One reporting sergeant told him: "Nigger, you're never going to finish your probation." Routine harassment ranged from buttons pulled off his uniform to cups of tea thrown in his face and the frequent slashing of the tyres of his car. Back in his police dormitory he cried bitter tears as fellow officers who had trained with him now refused to speak to him.

Three years of such abuse would end with his eventual transfer to another station, but not before his lowest moment, while on the beat out side the Royal Opera House in Covent Garden: "A police car drove past with three people in it. One stuck his head out of the window and shouted `Black c---.' I felt so ashamed."

That was then, this is now, and there was certainly a sense of shame among the audience watching Richard Norton-Taylor's play about the Lawrence case, The Colour of Justice, at the Tricycle Theatre recently.

The dramatised cross-examination of police officers laid bare an instinctive racism they seemed quite unable to recognise in themselves, and Jamie Acourt declared himself not at all shocked by the police surveillance video showing a number of the suspects using racist language and enacting violent fantasies.

Their mothers would later describe this as "bravado".

That "bravado" was prompted by television coverage of a football match: "Luke Knight complaining about the commentators wanting the Cameroons, `f---ing niggers' to win." Michael Mansfield QC put it to Jamie Acourt: "Your brother says `Makes you sick, doesn't it?'

Neil Acourt says, while picking up a knife from a window ledge and sticking it into the arms of a chair, "You rubber-lipped c--t. I reckon that every nigger should be chopped up, mate, and they should be left with nothing but f---ing stumps." Jamie Acourt hadn't forgotten, but was he shocked? "I ain't shocked. It is nothing to do with me."

Just as shocking for the audience, perhaps, and certainly more challenging, to hear Edmund Lawson QC question Linda Bethel, one of the first officers at the scene, about the failure to identify the source of Stephen's bleeding or to fetch the first aid kit from the police car pending the arrival of an ambulance.

He put it to her: "The suggestion has been made publicly that perhaps an explanation . . . was `police officers not wishing to dirty their hands with a black man's blood'."

A very ugly suggestion indeed, but the murder of the 16-year-old A-level student who dreamt of being an architect, the failure to bring anyone to account for it and the underlying contributory reasons for that failure add up to a decidedly ugly chapter in the history of policing in modern Britain. That such perceptions are held does not render them justified.

Most serving officers would be truly horrified by such an idea. Sir Paul, whose six-year career as Commissioner almost coincides with the story of Stephen Lawrence, declared racism in the force a No 1 priority soon after his appointment.

He rejected the charge of institutionalised racism at the Macpherson inquiry and has argued: "Definitions which suggest that most police officers are racist most of the time do no justice to the working lives and courage and integrity of police officers. Simplistic and pejorative labelling which obscures complex truth will not inspire, and we must inspire."

Sir Paul's rejection of the institutionalised racism charge is rooted in Lord Scarman's definition of that as "knowingly" and "as a matter of policy". However, he has also signalled his readiness to embrace a new definition, courtesy of the Macpherson report. He is unlikely to consider it a humiliating climbdown if, as has been suggested, that new definition allows that discrimination can result from "unwitting prejudice, ignorance, thoughtlessness and racist stereotyping".

That might not be enough for some Labour MPs, who believe Sir Paul, due anyway to retire next January, would not now have the credibility to launch a convincing programme of reforms. Much may hinge on the reaction of the Lawrence family and of the wider black community after today's publication of the report, and the government and police response to it.

However the signs are that the Home Secretary, Jack Straw, thinks Sir Paul's departure would deal a dangerous blow to police morale. There is a growing anxiety that today's criticisms should not be so sweeping as to encourage a victim culture among young blacks, or what one commentator yesterday described as "a defensive passivity among the police".

Macpherson's report and recommendations will be the most dramatic since Lord Scarman's on the Brixton riots. The government's and Sir Paul's responses are intended to be equally so. Together they promise a defining moment for policing, and for race relations in Britain.

As Roger Graef put it in a thoughtful article in the Evening Standard: "Police are trying to change attitudes within the force that have not changed in the wider society in which they live and work. As ever, we are asking more of them than we do of ourselves."