The UK police watchdog will carry out the country's biggest-ever investigation into police actions in response to last month's damning independent report into the 1989 Hillsborough disaster, in which 96 Liverpool fans lost their lives.
The Independent Police Complaints Commission (IPCC) and the director of public prosecutions will launch inquiries into possible crimes committed by police, meaning former and currently serving officers could face criminal charges.
The IPCC said the full-scale inquiry will examine the role played by officers on the day of the tragedy and the cover-up in its aftermath.
The Hillsborough Independent Panel's report revealed that 164 police statements were altered, 116 of them to remove or change negative comments about the policing of the FA Cup semi-final between Liverpool and Nottingham Forest.
Director of public prosecutions Keir Starmer will consider whether individuals or corporate bodies should face criminal charges after the inquiry has been completed and any evidence of criminal conduct referred to the Crown Prosecution Service.
Deborah Glass, the deputy chair of the IPCC, said the investigation would "without a shadow of a doubt" be the biggest carried out into police behaviour in the UK.
South Yorkshire Police will come under scrutiny for the policing of the disaster, while the West Midlands force, which led a 1989 external inquiry into officer behaviour at Sheffield Wednesday's stadium, will be investigated.
A South Yorkshire Police spokesman said: "South Yorkshire Police acknowledges the decisions today by the Independent Police Complaints Commission and the Director of Public Prosecutions following the release of the Hillsborough Independent Panel's report.
"The force will continue to co-operate fully with any judicial processes, as shown by the full co-operation with the panel during the three-year disclosure process.''
It is not yet clear how many individual officers will be investigated, but West Yorkshire Police Chief Constable Sir Norman Bettison, a chief inspector with the South Yorkshire force at the time of the disaster, will be one of them. Bettison, who announced last week that he would retire in March, was referred to the IPCC by West Yorkshire Police over allegations that he supplied misleading information in the aftermath of the disaster.
Glass, announcing the inquiry, added that Bettison will also be investigated over allegations that he "attempted to influence the decision-making process of the West Yorkshire Police Authority in connection with the referral they had made".
He has denied any wrongdoing, and said he was keen to co-operate with the inquiry.
A West Yorkshire Police spokesman said: "Sir Norman Bettison has consistently made the point since September 15, three days after the report was published, that these were matters that needed to be investigated formally and fairly by the IPCC.
"At the time, he immediately welcomed the police authority's decision to refer this matter. He is on record as saying he is keen to co-operate with the IPCC enquiry, but now that has been launched, he has nothing further to add."
Glass confirmed that the IPCC will examine whether police officers had committed perjury and perverted the course of justice following the disaster.
She said the inquiry will examine the circumstances surrounding the amendments to police statements and look at allegations that misleading information was passed to the media, politicians and previous inquiries in an attempt to deflect blame from the police onto Liverpool supporters.
The actions of police officers will come under scrutiny after the independent report revealed the blood alcohol levels of victims had been checked and Police National Computer checks on them undertaken in an attempt to sully their reputations.
Another element of the inquiry will focus on the role of West Midlands Police and its 1989 investigation into the disaster, and Chris Sims, the chief constable of the force, welcomed the IPCC's decision to announce the inquiry.
He said: "As chief constable, I read the Hillsborough Independent Panel report and immediately referred West Midlands Police to the IPCC. Today, the IPCC have accepted that referral, and West Midlands Police will give their full support to the IPCC investigation."
The Attorney General, Dominic Grieve, will consider whether to apply to the High Court to quash the original inquest verdicts on the 96 people who died and seek new inquests.
Glass said: "Justice demands that we do whatever is possible to investigate culpability for any offence that may have been committed, and to do so thoroughly and fairly.
"The families [of the dead] have already waited for 23 years. I want to give them my assurance that we will do everything in our power to investigate these serious and disturbing allegations with the careful and robust scrutiny they deserve."
Margaret Aspinall, the chair of the Hillsborough Family Support Group (HFSC), told BBC News that she welcomed the investigation, saying: "I'm very pleased with what I've heard so far. We've had the truth, now we're starting the search for justice.
"We have more waiting ahead of us, but we have had 23 years of waiting. I'm hoping it's going to be sooner rather than later, as I'm a lot older now. I was a relatively young woman at the time. I'm now an old age pensioner."
Anne Williams, whose 15-year-old son Kevin died in the disaster, said: "This is really good news – good news that they are moving quickly to save us any more heartache. I welcome it. I know it's going to be a long process, but at least they are getting the ball moving."
The Mayor of Liverpool, Joe Anderson, said the announcement "sets the wheels in motion at long last" and welcomed the news that former and serving South Yorkshire Police officers including Bettison will be investigated.
He added: "The part he and others played, revealed by the independent panel's report, showed that there must be a full investigation into their role in the cover-up.
"I sincerely hope that no stone will be left unturned in delivering the outcome that the families deserve."