One in six people who had made a will admitted that it was out of date, in a survey of 1,600 adults conducted by Mintel International. And many others may be unaware that changes in the law have altered the effects of their wills, according to solicitors Taylor Joynson Garrett.
The Mintel research suggests that large proportions of the population are failing to keep their wills up to date when their circumstances change.
A man who divorces and remarries, for example, without updating his will would still, in most circumstances, leave an inheritance to his former wife and nothing to his present wife. Children and grandchildren can similarly be bypassed.
Paul Hersey, the senior finance analyst at Mintel, said: 'By having an out-of-date will you could be cutting out someone you want to support after your death. If the will is left unrevised you may be causing more problems for your executors and heirs than if you had died intestate.'
Meanwhile, Taylor Joynson Garrett believes that many people - women in particular - who now believe they stand to inherit large sums from their spouses could instead find that their stake is bypassed in favour of their children.
Farmers, Lloyd's underwriting members and owners of family businesses are those most at risk.
The problem stems from the change announced in last year's Budget, which saw Inheritance Tax relief increase from 50 to 100 per cent on certain business and agricultural assets - particularly farms and majority shareholdings in family businesses. (The relief on minority shareholdings in family businesses rose from 30 to 50 per cent.)
According to Philippa Blake-Roberts of Taylor Joynson Garrett, it has been standard practice for wills to include direct references to the principles of the law. Therefore, many wills say that assets on which Inheritance Tax is potentially chargeable should be passed to the spouse - who will inherit them tax-free - while assets on which there is relief should go to other beneficiaries, typically the children or grandchildren.
Many farmers' wives, for example, could find that instead of inheriting half of their husband's farm, they are dependent on the charity of their children. 'The danger is that these wills won't get reviewed before the people die,' Ms Blake-Roberts said.
Help the Aged has just published a free information pack to help people make a will. Copies are available from Legal Department, Help the Aged, St James's Walk, London EC1R OBE.