Changes to the rights of people whose spouses or civil partners die without making a will have come into force.
For married couples with no children, the surviving partner will now inherit their spouse's entire estate, rather than £450,000 then half of the rest.
For those with children, the surviving spouse will get the first £250,000 as before, but they will now get half of the remainder - rather than just interest on that amount.
The changes apply in England and Wales.
BBC legal correspondent Clive Coleman said the law governing inheritance when one spouse dies without making a will had "changed little in the last century".
He said the changes were designed to make the system fairer and address concerns that is has been "difficult for those who deserve an inheritance to get one".
More legal disputes?
The new rules, made law by the Inheritance and Trustees' Powers Act, include increased inheritance rights for people who were effectively stepchildren of the deceased even though their parent was not married to them.
The changes remove a rule which meant children lost their right to inherit from a parent's estate when they turned 18 if they were adopted by another family before that age
Under former rules, where no will was left the surviving spouse in a childless couple got the first £450,000 then half of anything above that, with the rest divided among other relatives.
In cases where married couples had children, the surviving spouse was only entitled to interest on their half of anything over £250,000 under the old rules - with the sum itself given to the children after the surviving spouse died.
Paula Myers, of law firm Irwin Mitchell, said: "While much of the new legislation undoubtedly brings the law on wills into the 21st Century, there are aspects which raise questions and could in the long term lead to an increase in the number of legal disputes launched regarding inheritance issues."
The firm said these disputes could include claims from children who would have received more of their parents' estate under the old rules.