The person’s next of kin - eg the spouse (or civil partner) or children - can usually apply for a grant of representation.
The law decides who inherits the estate if there’s no will.
Partners and ex-partners
If you’re the partner of the person who’s died but weren’t married to them (or in a civil partnership) you can’t apply. You’re also not automatically entitled to get any of your partner’s estate. You should get legal advice to find out about your rights.
If the person was separated but not yet divorced (or had their civil partnership dissolved), their spouse (or civil partner) would inherit some or all of the estate - and must apply for the grant.