More than half of UK adults haven't made a will and 10% of them say writing one has never even occurred to them.
The figures from Unbiased.co.uk and Certainty.co.uk also show that more than two thirds of adults in their 40s don't have a will, despite many of them having young families to support.
Regionally, Northern Ireland has the most adults without a will (68%), whereas in the South West of England this drops to 48%.
The high proportion of people not having a will is at odds with the rising number of people who expect to leave money and property to their loved ones.
Indeed, 76% now plan to leave money to their families - up from 74% last year - with an average value of £46,000. While 68% hope to leave property - up from 65% in 2012 - with an average expected value of £202,000.
However, without a valid will those assets may not end up with the intended beneficiaries.
Karen Barrett, chief executive of unbiased.co.uk, explained: "Too few people are aware of the security a will offers and too many believe it should be written when they are older. Writing a will is about protecting loved ones, making sure they receive exactly what is intended for them and ultimately achieving control over your finances, even when you are no longer around."
Unbiased.co.uk and Certainty.co.uk are teaming up for their fifth annual 'Write and Register a Will Week' from Monday 14 October, which waives the usual will registration fee of £30.
However, November will see the arrival of Will Aid 2013 when local solicitors write your will - but instead of paying their fee you are invited to make a donation to Will Aid.
The suggested donation is £90 for a basic single Will or £135 for a pair of basic mirror Wills. You can find participating solicitors at willaid.org.uk/will-makers/find-solicitor.