Prince Harry will attend a memorial on Monday marking the 25th anniversary of the racist murder of black teenager Stephen Lawrence in a killing that triggered far-reaching changes to British attitudes and policing.

The prince and his fiancee Meghan Markle will join Stephen's mother Doreen Lawrence, who campaigned tirelessly for justice after her son was brutally stabbed to death at a bus stop on April 22, 1993.

Five suspects were arrested within days but failures in the police probe into the 18-year-old's killing meant it was not until 2012 that two of them were jailed, after new evidence came to light.

The case drew the interest of Nelson Mandela, who met the Lawrence family a fortnight after Stephen died, telling them: "It seems black lives are cheap."

A public inquiry in 1999 found the initial police investigation was marred by incompetence, a failure of leadership and "institutional racism".

It led to an overhaul of policing in Britain, with London police chief Cressida Dick this weekend noting the "huge and positive change" enacted.

However Jon Boutcher, a senior officer who leads on race issues for the National Police Chiefs' Council, warned that concerns remain in some communities.

"Because of the legacy of the past, we must accept some still view policing as institutionally racist and we need to work doubly hard to gain trust," he said.

It has since emerged that the police also sent an undercover officer to spy on the grieving Lawrence family in the late 1990s as they campaigned for justice.

The National Crime Agency (NCA) meanwhile is still investigating allegations of corruption in the case.

Father forgives 

Harry, who is due to marry US actress Markle on May 19, will read out a message of support on behalf of his father Prince Charles at the memorial at St Martin-in-the-Fields Church in central London.

Stephen had ambitions to become an architect when he was set upon by five white youths at a bus stop in Eltham, southeast London, in what an inquest ruled was an unprovoked racist attack.

Despite the early arrests, state prosecutors initially concluded there was insufficient evidence to progress with murder charges against any of the five suspects.

Two of them, Gary Dobson and David Norris, were finally convicted in January 2012 on the basis of new forensic evidence and jailed for 15 and 14 years respectively.

Stephen's father Neville, who split with his wife after the murder and returned to his home country of Jamaica, said earlier this month that he has now forgiven the men who killed his son.

"I have justice and I have freedom from the burden that I have carried for so many years, through forgiveness," he said.

Doreen Lawrence, who set up a youth charity in her son's name and was in 2013 appointed to the House of Lords, has also said that it is now time to move on.

"25 years is a long time to be out campaigning," she told ITV television, adding that she would like to spend more time "on positive things", including her grandchildren.

"For me, I've not really lived my life in the way that I would have liked to. I'm always being asked to do something. Now I think, enough."