Millions of people are risking the future financial security of their families by not having a will, a consumer group claims.
The National Consumer Council said co-habiting couples who are not married can lose property and cash if a partner dies before putting their estate in order.
And parents are "gambling" with the long-term prospects for their children if they refuse to plan ahead and make a will, the group added.
Apathy is the main reason why most people do not put together a contingency plan in the event of their own death, the survey of 2,673 people in England and Wales found.
But the NCC claims a "wills time-bomb" is looming, and is now urging the Government to encourage more people to organise their affairs.
More than 83% of couples who live together have not made a will, its research found.
This is roughly the same among households with dependent children.
NCC spokesman Steve Brooker said: "Dying without leaving a will can leave all sorts of headaches for those left behind. It can create family feuds and leave relatives short of their inheritance."
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