A legal principle which prevents people being tried for the same crime twice has been scrapped in England and Wales.
Police are to re-examine the murder of Julie Hogg
The ban on "double jeopardy", which has existed for around 800 years, took effect from Monday.
The Court of Appeal can now quash an acquittal and order a retrial when "new and compelling" evidence is produced.
Police plan to re-examine the case of 22-year-old Julie Hogg, who was murdered in a sex attack at her home in
Billingham, Teesside, in November 1989.
Boyfriend Billy Dunlop was tried for the murder of the pizza delivery girl, but acquitted after the jury failed to reach a verdict on two separate occasions.
The change will apply retrospectively, so someone could face a second trial if evidence such as DNA material, new witnesses or a confession came to light.
A Home Office spokesman said: "It is important the public should have full confidence in the ability of the criminal justice system to deliver justice.
"This can be undermined if it is not possible to convict offenders for very serious crimes where there is strong and viable evidence of their guilt."
Director of Public Prosecutions Ken Macdonald expects no more than "a handful" of cases to be brought a year.
A Crown Prosecution Service spokeswoman said: "There has to be new evidence which was not available at the time of the original trial.
"Just because someone is reported to have confessed - in a book or a newspaper interview - does not necessarily mean that is evidence in a form we could use."
The National Crime Faculty believes there are 35 murder cases in which acquitted defendants could be re-investigated and new charges brought.
The reforms - which also allow hearsay evidence to be admissible in court - come under the new Criminal Justice act.
They apply to 30 serious crimes - including murder, rape, Class A drug offences and war crimes - but double jeopardy remains in force for lesser offences.
The Bar Council's former chairman Matthias Kelly QC said the law changes could "lead to prosecutions routinely seeking a second bite of the cherry, if a case flopped first time for good reason".
And civil liberties groups also condemned the move, fearing the law could be used to persecute people and lead to miscarriages of justice.
However, it will only be possible to retry an acquitted person once.
The Stephen Lawrence racist murder case is another episode where detectives hope new evidence could come to light.
Other cases could include Ronnie Knight, the ex-husband of EastEnders actress Barbara Windsor, and ex-Kray Twins associate Freddie Foreman.
Both have written books where they allegedly confess to involvement in murders.