Helen’s Law has been granted Royal Assent and will come into force within weeks.
The legislation is named after Helen McCourt, the 22-year-old murdered by pub landlord Ian Simms in 1988.
Her remains have never been found and mother Marie, 77, campaigned to halt the parole of killers who refuse to reveal where their victims’ bodies are.
The Prisoners (Disclosure of Information About Victims) Bill was approved in the House of Lords earlier this week and Royal Assent was agreed on Wednesday.
Marie said: “This has been a long, gruelling fight and there were times I doubted it would happen.
“But knowing Helen’s Law has been signed off by the Queen and will shortly go on to the statute books has made me the proudest mum in the world.
“It is a lasting legacy to my lovely daughter.”
Justice Secretary Robert Buckland QC added: “Thanks to the efforts of Marie McCourt and other campaigners, more families should get the answers and closure they deserve.”
Marie’s victory is bittersweet as Simms, 63, walked free earlier this year. He killed insurance clerk Helen as she walked home from work in Billinge, Wigan.
Marie launched her fight in the Mirror in 2015 and hundreds of thousands signed petitions.
Mrs McCourt’s husband John Sandwell said: “We are so relieved after five years – it has been a long old fight and very difficult times with the amount of obstacles we have had to go through.
“But we stuck with it and with the support of people like the press and the Commons and the Lords, we finally we have got there, we are delighted.
“We were hoping that the new law would have caught Simms before he walked but it came just too late.
“It causes a lot of frustration and heartache but we never gave up.
“At least it’s going to be able to help other families and it will be a legacy for Helen. It’s a law that will help an awful lot of families in the future.”
He said that until Simms reveals the whereabouts of Helen’s body, he still “has that control over our lives”, adding: “He has that string he can pull like a puppet almost.”
February 1988 Helen, 22, disappears on way home from work. Pub landlord Ian Simms charged with murder. Search for her remains begins.
February 1989 Simms found guilty of murder in first conviction secured without a body using DNA. He gets life with a minimum tariff of 16 years.
1991 Marie writes to Simms asking for information on Helen’s whereabouts. His reply is abusive.
October 2013 Police exhume a grave in St Aidan’s Churchyard, Billinge, but Helen is not there.
December 2015 Simms applies for parole, prompting Marie to launch her campaign. Her petition has been signed by almost 600,000 people.
February 2016 Parole Board recommends that Simms is moved to an open prison.
October 2016 MP Conor McGinn’s Bill on Helen’s Law is passed unanimously at first reading.
February 2018 Western Australia introduces “No Body, No Parole” laws.
March 2019 Simms is spotted in Birmingham on temporary release from open prison.
May 2019 Marie meets David Gauke.
Simms, a pub landlord who was convicted by a jury on overwhelming DNA evidence of the 22-year-old’s abduction and murder, has always maintained his innocence.
MPs voted in favour of the law in 2016 but it did not receive Government backing until last year. It was then presented to Parliament after being mentioned in the Queen’s Speech.
But the Bill had to be scrapped amid the general election and the prorogation of Parliament, before being brought back for debate for a second time and eventually approved.
Parole Board guidance already says offenders who withhold information may still pose a risk to the public and could therefore face longer in prison.
Courts can also hand down tougher sentences for murderers who deliberately conceal the location of a body.
But the Bill makes it a legal requirement for the Parole Board to take into account a killer’s failure to disclose the location of their victim’s remains when considering them for release.