Hundreds of families are expected to benefit from a change in the law which comes into force from today, allowing them to take control of their missing loved ones' financial affairs.
Claudia's Law is named after York chef Claudia Lawrence, who vanished without a trace a decade ago.
Police believe the 35-year-old from York - who has not been seen since March 18 2009 - was murdered but her body has never been found.
Her father Peter Lawrence, who has campaigned tirelessly for a change in the law ever since his daughter's disappearance, said "several hundred families" were "queuing up" to make use of the legislation known as the Guardianship (Missing Person's) Act 2017.
He said "It's taken an awful long time but the important thing is it's now here.
"When people are at their lowest emotional ebb, they suddenly find that they can't deal with all these financial and practical things. Well, now that's over.
"There are several hundred families queuing up to deal with this and the Government reckon there might be about 50 applications to the court a year after that.
"It's really important and thank God it's here.
"Claudia is still missing. Obviously the fact we don't know what's happened to her is distressing.
"Let's hope for everybody else's sakes that they are able to deal with their families' affairs."
Under the law, families can apply to the High Court for guardianship of the affairs of a missing person after they have disappeared for 90 days or longer.
It will mean they can handle everyday financial matters such as making mortgage payments and suspending direct debits for bills.
Operated by the Office of the Public Guardian, families will be able to make use of the scheme for up to four years before having the option of renewing the legal status.
Previously, families could only take over the financial affairs of a missing person if they were declared dead.
Mr Lawrence's friend and spokesman Martin Dales praised the work he and many others have done to ensure a change in the law was introduced.
Mr Dales said: “Peter, along with the charity Missing People and parliamentarians such as MPs Kevin Hollinrake and Julian Sturdy and Peers including Baroness Sally Hamwee and Ministry of Justice ministers and officials, have worked tirelessly in pressing the Government of the urgency for this legislation.
“Peter has to be admired for his strength and fortitude in coping with the stresses and strains of getting brand new legislation onto the statute books amidst him, the family and friends emotional pain in waiting all these ten-and-a-half years for news of Claudia's whereabouts.
"It has been a long journey and will continue until Claudia is found but today is a cause for celebration amidst the pain and sadness.”
Susannah Drury, director of policy and research at the charity Missing People, branded the law a "triumph" for campaigners.
She said: "This regulation will mean that families who face the emotional distress of a disappearance will not be blocked from handling the financial and legal affairs of their loved ones."
Advice on applying for guardianship will be available through the charity, Mr Lawrence said.