What is it?
The change in the law means families of missing loved ones will be able to take control of their financial affairs in their absence.
Campaigners say this is a huge step forward in recognising the responsibilities placed on relatives when someone disappears and the current legal difficulties in obtaining permission to act on their behalf.
Previously, families could only take over the financial affairs of a missing person if they were declared dead under the Presumption of Death Act 2013.
The legislation is named after chef Claudia Lawrence, who vanished without 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 has campaigned tirelessly for changes to the law since her disappearance.
What does the law mean?
Families can apply to the High Court for guardianship of the affairs of a missing person after they have been gone for 90 days or longer.
It will mean they can handle everyday financial matters like making mortgage payments and suspending direct debits for bills.
Operated by the Office of the Public Guardian, families will be able to use the scheme for up to four years before having the option of renewing the legal status.
What will families need to do to obtain guardianship?
The applicant will have to satisfy certain conditions before becoming a guardian of assets.
Applicants will have to provide evidence the person is missing and their credentials, supporting information to show they have not been seen for 90 days and a witness statement.
The application will be made to the Chancery Division or the Family Division of the High Court using a "part 8 claim form", which is available online.
Fees start at £200 to register and means-tested financial support is available to applicants, the Ministry of Justice (MoJ) said.
The Guardianship (Missing Persons) Act 2017 Code of Practice has more detailed information and the MoJ said it will publish a step-by-step guide on gov.uk.
What does the Government say?
A Ministry of Justice spokesperson said: “Coping with the tragedy of a loved one going missing can cause immense pain and stress.
“Claudia’s Law, campaigned for so tirelessly by Peter Lawrence, will mean that from today families can manage their loved one’s affairs in their absence – removing an enormous burden at such a distressing time.”