Bernard Hogan-Howe says he has been 'shocked' by recent revelations and 'will not stand for any racism or racists'

The Metropolitan police commissioner, Bernard Hogan-Howe, has urged his staff to report inappropriate behaviour by colleagues in the wake of the Guardian's investigation into alleged racism in the force.

In a personal address to Scotland Yard's 48,000 staff, Hogan-Howe said he had been "shocked" by the revelations and added: "I will not stand for any racism or racists."

"We have a duty to challenge or report any behaviour by colleagues which is less than the high standard demanded by the service and Londoners themselves," he said. "You cannot avoid that duty. Nor can I."

The message was released as the Crown Prosecution Service announced it would conduct a second urgent review, amid growing controversy over the advice its lawyers have given about the possibility of prosecuting police officers.

The commissioner said: "What we have heard over the last few days is of course completely at odds with all the close work that goes on with communities across London every day. We work hard to forge strong relationships with people who can advise us and rightly hold us to account."

On Friday, the Guardian released an audio recording from a mobile phone in which a police officer could be heard using offensive racist slurs against a black man arrested after the riots.

The CPS initially decided not to bring charges against the officer who made the remarks, PC Alex MacFarlane. Senior lawyers at the CPS are now reviewing the case and MacFarlane has been suspended.

The controversy deepened on Monday after the Guardian revealed that one of the officers present when the racist remarks were made, PC Joe Harrington, allegedly assaulted another black man, aged 15, hours later. Harrington, who was subject of an investigation, has been placed on restricted duties and the Met is considering what, if any, disciplinary action to take against him.

The CPS announced on Tuesday it would appoint a senior lawyer to review advice given to the Independent Police Complaints Commission that there was no realistic prospect of convicting Harrington for assault.

"I have now directed that a review of this case be conducted as a matter of urgency," said Grace Ononiwu, deputy chief crown prosecutor for the CPS London. "The CPS is now in the process of obtaining all the evidence in this case, including some that was not previously considered. A senior lawyer with no previous involvement will complete this review as soon as is practicable."

The CPS now has two urgent reviews underway into advice given by Carl Kelvin, a lawyer at the CPS police complaints department. In both cases Kelvin advised the IPCC against prosecuting Newham police officers accused of racially abusing or assaulting young black men on 11 August 2011 – the day after the London riots ended.

Hogan-Howe, who is Britain's most senior police officer, is known to have told colleagues he was appalled after listening to the recording over the weekend.

His broadcast urging staff to report bad behaviour by colleagues was made available to the Met's 35,000 police officers and community support officers and 13,000 civilian staff."Today I want to speak to you about one subject," he said. "I'm sure that you've all been as shocked as I have been about the recent reports of apparent racist remarks and alleged assault by some officers in Newham in August of last year. Even so, I wanted to take this opportunity to clearly and categorically reiterate that there is no place for racism in the Met."

He added: "I know that the Met has changed greatly over the years, and most of you have directly been involved in improving our relationship with all Londoners and those who visit us. Unfortunately just one alleged incident like this can be very damaging to public confidence.

"This is a great organisation – one we should all be proud to work for. Without pre-judging this case, in any instance of wrongdoing it is a small few who tarnish the vast majority."

The statement continued: "What we have heard over the last few days is of course completely at odds with all the close work that goes on with communities across London every day. We work hard to forge strong relationships with people who can advise us and rightly hold us to account.

"I want us all to go out there and remind those communities of our commitment to fairness and professionalism. I will not stand for any racism or racists in the Met."