UK police chiefs have pledged to stamp out racism in their forces after five officers were forced to resign after being exposed in a BBC sting operation.

In a joint  letter published on Thursday morning in a London daily newspaper, the president of the Association of Chief Police Officers, the chief constable of Greater Manchester Police, the deputy commissioner of the Metropolitan Police Force in London, the chief constable of the Northern Ireland and 40 other chief constables declared: "There is no room in the British police service for anyone with racist attitudes."

Their letter follows last Tuesday's screening of a BBC television documentary, 'The Secret Policeman', that has prompted a criminal investigation into remarks made by eight officers to undercover reporter Mark Daly.

Rob Pulling, one of eight officers featured in the documentary, has since resigned from the North Wales Police. A top officer in his force admitted having felt 'physically sick' watching the film.

Three other officers from Greater Manchester Police and one from the Cheshire force have also stepped down.

All five officers had earlier been suspended over allegations of racist behaviour captured on secret cameras for the hour-long documentary.

Before the programme was screened, the BBC was criticised by UK Home Secretary David Blunkett for not handing over evidence of racism before broadcasting its documentary.

Subsequently, Blunkett added his voice to the many senior police officers who admitted feeling sick and ashamed by the extreme racist behaviour of young recruits.

Shadow home secretary Oliver Letwin said Blunkett was wrong to attack what had turned out to be a "very useful and important piece of BBC journalism."

He said: "It is pretty clear, alas, that something has to be done to check, not only during recruitment but also after it, that this sort of thing is not going on. I think what this shows is that despite the alarming state of affairs revealed by the ghastly Stephen Lawrence episode, not enough has been done."

'The Secret Policeman' showed racism among a number of trainee officers from forces in North Wales, Cheshire and Greater Manchester after reporter Daly posed as a recruit.

The Glaswegian journalist spent seven months with Greater Manchester Police compiling a dossier of evidence against seven recruits and one serving officer before being exposed and arrested.

PC Pulling, based in Rhyl, North Wales, was seen wearing a home-made Ku Klux Klan-style hood, saying he would bury an Asian under a train track and that Hitler had the 'right idea'.

Murdered black teenager Stephen Lawrence had 'deserved it' and his parents were "a f****** pair of spongers', he added.

Acting deputy chief constable of North Wales Police Clive Wolfendale commented: "It is frankly hard to imagine more despicable words."

Chief Constable Richard Brunstrom would now be writing to the Lawrence family to apologise.

Wolfendale said it was 'scant comfort' that 'raw recruit' PC Pulling had never walked the streets of North Wales unsupervised.

"I felt physically sick as I watched 'The Secret Policeman' last night and I suspect some of you did. Pulling has shamed his colleagues, his uniform and his service. He is a disgrace," he said.

Doreen Lawrence, mother of murdered Stephen Lawrence, told reporters: "I found it quite frightening, I was numb for ages after watching it. It was quite a shock to know that after all this time that people still held those views."

Greater Manchester Deputy Chief Constable Alan Green said any case the racist recruits were involved in would have to be reviewed.

"It is fair to say the programme last night, which we had not seen previously, has really shocked, certainly, me. It has made me ashamed to be part of the British police service and it has saddened me greatly."

Daly was arrested on suspicion of obtaining a pecuniary advantage by deception and damaging police property. The charges are understood to relate to drawing a police recruit's salary while working as a journalist and hiding a camera in body armour.

He was due to answer bail next month but Mr Green hinted that the reporter would not face charges.