The British government has announced an annual memorial day in honour of a murdered teenager, Stephen Lawrence, who was killed in a racist attack near his south London home 25 years ago. Stephen Lawrence was born to Jamaican immigrants to the UK. He died at the age of 18 of stab wounds after being attacked by a gang of white youths while he waited at a bus stop on 22 April 1993. A formal independent public inquiry, led by retired High Court Judge Sir William Macpherson, was held into the police investigation and subsequent events. Archbishop Sentamu was one of three independent advisors appointed to the Inquiry, which found “institutional racism” within the Metropolitan Police.

“Within a few hours, somebody turned up to the police with the names of the people who had killed Stephen and had said they had hidden their knives under floorboards,” Archbishop Sentamu told BBC Radio Four’s Sunday Programme this weekend. “And of course, as we did hear in the inquiry, the police did never . . . lifted up the floorboards to find whether these knives where there.”

The family of Stephen Lawrence launched a private prosecution against the five individuals, after the Crown Prosecution Service said that there was insufficient evidence. The five were acquitted. A change in the law, which came about as a direct result of this case, removed the “double-jeopardy” rule which prevented a person from being tried twice for the same offence. As a result, two of the five are now serving life imprisonment after a 2012 trial. Police say that the case remains open, should further evidence come to light.

The case shone a light on the extent of the undercurrents of racism experienced by ethnic minorities in the UK. Archbishop Sentamu told the Sunday Programme that he, too, had suffered racism in south London. The first funeral he had taken as a curate was of a young boy. “The father of the child came out and said: ‘what crime did my son commit to be buried by a black monkey?’ That was the first. And there were others who said that they didn’t want me to take their funerals because I was a black person. But my vicar was very robust and said that if they didn’t want me to take their funerals, he wasn’t going to do it either. He stood by me.

“Then another lady didn’t want me to do a funeral and she went around from church to church and eventually had to come back to me.” He also revealed he was stopped by the police 28 times under “stop and search” powers that were disproportionately used against black people – the last time was when he was Bishop of Stepney and was stopped in his car. “I opened my bonnet, I opened the boot. The gentleman got very aggressive, actually, and then realised I had a dog collar on and said ‘whoops’! So I have experience of all this harassment.”

He said that the evidence he heard as part of the Independent Inquiry panel was “quite shocking” and led the team to define what racism and institutional racism was. “The word ‘racism’ can be bandied about without giving it context,” he said. “We said it consists of conduct, words or practice which disadvantage or advantages people because of their colour, culture, or ethnic origin; and in its more subtle form it is as damaging as its more overt form.”

He said that institutional racism was “the failings of an organisation collectively to provide an appropriate and professional service to people because, again, of their colour, culture, or ethnic origin. And you can see it in its processes, attitudes, behaviour, which amount to discrimination through unwitting prejudice, thoughtlessness and stereotyping. I had been stereotyped: because I was black there was an assumption that I must have committed some crime.”

He called on every police force in the UK to use the 25th anniversary of the murder of Stephen Lawrence to review the Inquiry’s 72 recommendations to see how they measure up.

On Monday, Stephen Lawrence’s family joined around 800 people at a memorial service at St Martin in the Fields, the high-profile Church in London’s Trafalgar Square. The service was attended by Prince Harry and his fiancé Meghan Markel, the British Prime Minister Theresa May, Opposition Leader Jeremy Corbyn, and the Commissioner of the Metropolitan Police, Cressida Dick. Mrs May used the occasion to announce that the anniversary would be commemorated annually in the UK as the Stephen Lawrence Memorial Day.