The city of Liverpool will come to a standstill today as it remembers the 96 lives lost in the Hillsborough disaster 25 years ago.
The jury in the inquest to the Hillsborough tragedy is to make a visit to the stadium - but should not read the "deeply moving" tributes.
Hillsborough coroner promises new inquests will try to expose "any culpable or discreditable conduct".
It said in a statement Scotland Yard said:
_ "We have been asked in recent days whether undercover officers were deployed into the Hillsborough campaign._
"We replied that we will neither confirm nor deny details of the deployment of undercover officers. This is a long-established practice to avoid criminals targeted for undercover operations drawing conclusions if we were to give negative answers in some cases but not comment in others.
"Once we start denying false or incorrect allegations, our silence in other cases could be taken as a confirmation, and that could be very damaging and dangerous for those who risk their lives to combat organised and serious criminality.
"The 'neither confirm nor deny' policy is a practice we also follow in the courts when asked whether an individual was an informant or an undercover officer, and judges have accepted its legitimacy in the interests of the wider public good.
"It may look strange in an era of greater transparency and accountability, but we believe this principle continues to protect those who take the greatest risks on all our behalf."
"We would like to explain that the reason why under the Freedom of Information Act, we could not provide an answer, was to protect national security and undercover police tactics. The language used was relevant to those issues and was not a comment upon any of the Hillsborough Groups.
"We note that no-one has drawn to our attention any evidence that has been published to suggest that undercover officers were deployed over Hillsborough."
Scotland Yard will remain silent over claims that undercover officers spied on Hillsborough campaigners "to protect national security and undercover police tactics", the force has said.
It refused a Freedom of Information request by the magazine Private Eye for files on the Hillsborough Justice Campaign and the Hillsborough Family Support Group amid claims the campaigners were put under surveillance.
The force has a policy to neither confirm nor deny what police moles have been up to, in a bid to protect officers who work undercover.
Scotland Yard said the Private Eye request was refused to protect tactics.
The "neither confirm nor deny" approach prompted anger among bereaved parents trying to discover whether their dead children's names had been used by undercover officers.
Last summer Jules Carey, solicitor for Barbara Shaw who believes that her son Rod Richardson's name was used, said she felt her complaint to Scotland Yard had been "swept under the carpet".
A report found that the names of 42 dead children were used by officers trying to create watertight false identities.
The lawyer acting on behalf of the families of those who died in the Hillsborough disaster has called for the inquest to be televised.
Speaking at a pre inquest hearing in London today, Michael Mansfield QC, urged the coroner, Lord Justice Goldring, to allow the proceedings to be shown to the public.
The families also heard around three thousand images of the disaster have been prepared, and pathology evidence will assess the time of death for each of the ninety six victims. The full hearing should start before the end of March next year.
A barrister has told a pre-inquest hearing that handheld footage filmed by police at Hillsborough may have been edited.
Pete Weatherby QC, who is representing 21 victims' families, asked for an audiovisual expert to be among those to give evidence to the inquest to ensure that the best possible copies were shown to the jury.
The Hillsborough Preliminary hearing has revealed more police statements from the stadium disaster had been changed than previously thought.
The Independent Police Complaints Commission investigation show 238 police statements have been altered, originally it was thought to have been 164.
220 of those officers are still alive. 132 of those officers have been invited for interview.
Lord Justice Goldring says he "remains committed" to a start date of March 2014 for Hillsborough inquests
The families of Hillsbrough victims have unfurled a banner outside the preliminary hearing in London in honour of Anne Williams.
The campaigner who died in April fought for justice for her 15 year old son Kevin who killed at the disaster.
The other side shows Sir Norman Bettison who is under investigation over claims he was behind a police cover-up.
Barry Devonside arrives at the Hillsborough preliminary inquest hearing in London.
His 18-year-old son Christopher was one of the 96 Liverpool fans to die in the disaster.
Families of ninety-six people killed in the Hillsborough disaster will today attend the final preliminary hearing ahead of fresh inquests next year. It's after the 1991 verdicts of accidental death were quashed by the High Court.
More than 90 police notebooks that could contain crucial information about the Hillsborough disaster have been recovered. They've been handed into South Yorkshire Police by retired and serving officers.
The force is the subject of the biggest ever inquiry into police conduct in the UK. The IPCC says it's already found evidence to suggest statements about the tragedy may have been changed.