A police officer will face prosecution over allegations he assaulted a 15-year-old black teenager who was handcuffed in the custody area of a London police station shortly after last summer's riots.

Joe Harrington, a Metropolitan police constable, is to be charged with assault "occasioning actual bodily harm" over allegations he attacked Terelle Ferguson, now 16, at Forest Gate police station.

The decision to charge the officer was announced by the Crown Prosecution Service, which had previously decided there was no realistic prospect of prosecution in the case. Alison Saunders, the chief crown prosecutor for London, said it was regrettable that prosecutors previously came to the wrong conclusion in the case.

"I hope the seriousness with which the CPS has taken this matter gives the public confidence that we are an organisation which will review our decisions, openly accept when we've got them wrong and then take the correct course of action," she said.

The complaint against Harrington, first reported in the Guardian, was one of 13 alleged racism cases identified by the Met in recent weeks. However, there was no suggestion in the CPS announcement that there was any racial dimension to Harrington's prosecution.

The decision to prosecute Harrington is the second time in a month the CPS has reversed a decision not to prosecute a police officer.

Saunders said: "In February this year, the Crown Prosecution Service decided there was insufficient evidence to charge PC Joseph Harrington in relation to an alleged assault on a young man in custody in August 2011.

"The incident was investigated by the Independent Police Complaints Commission, with CPS involvement beginning in November 2011. The IPCC gave summaries of interviews to the CPS in February, and in the same month the CPS decided that there was no realistic prospect of conviction in the matter.

"Grace Ononiwu, deputy chief crown prosecutor for CPS London, directed in April that there should be a new review of this case after concerns were raised with CPS London. That review is now complete and I have decided that there is sufficient evidence for a realistic prospect of conviction to charge PC Harrington with assault occasioning actual bodily harm contrary to section 47 of the Offences Against the Person Act 1861."

She added: "Occasionally, a new look at a prosecution decision shows that the wrong view of the evidence was taken. That is regrettable. In this case, the conclusion originally reached was not tenable on the available evidence.

"When a review shows a previous decision not to prosecute is clearly wrong, it is open to the CPS to rectify that error by reconsidering the prosecution decision at chief crown prosecutor level.

"As chief crown prosecutor for London, I have taken the decision in this case that not only is there sufficient evidence to provide for a realistic prospect of conviction, and that a prosecution is required in the public interest, but that a prosecution is necessary in order to maintain confidence in the criminal justice system.

"That is the test I must apply under the code for crown prosecutors when reinstituting a prosecution. It is clear that the allegation of using excessive force on a handcuffed 15-year-old in custody is a serious matter."

Saunders said she had advised the IPCC to summons Harrington. She added: "All parties have now been informed. Can I please remind all concerned that PC Harrington is to be prosecuted for a criminal offence and has a right to a fair trial. It is very important that nothing is said, or reported, which could prejudice his trial. Proceedings are now active."