A poll of 2,000 adults found 25 per cent had more than 200 worth of films, video and music stored online.
Nearly a third considered the sum valuable enough to be passed on to loved ones and 11 per cent have already put internet passwords in their wills.
Without the passwords being included in wills, billions of pounds' worth of films, music and pictures stored in "cloud" services such as Hotmail, Facebook, iCloud and Flickr would be lost.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
The research, by cloud computing company Rackspace, found that 53 per cent of those polled held "treasured possessions" in such services.
A quarter said they had "special photos" stored online, one in 10 had treasured videos and the same number kept sentimental emails from loved ones.
Lawyers described the passing on of valuable passwords as a major change to the traditional way wills have been drawn up.
Matthew Strain, partner at London law firm Strain Keville, said: "With more photos, books, music and so on being stored online and in digital format, the question of what happens to these when people are gone becomes more important every day.
"Online possessions - from digital photos and videos to music and apps - have monetary and emotional value to their owners, and potentially their loved ones.
"People have not yet come to grips with the value of these digital possessions and the risk is that they may be lost if the owner dies, or even that their estate may be liable for ongoing subscriptions to online magazines or newspapers, for instance.
"We have started to advise clients on the topic of digital inheritance as it is something people should be thinking and doing something about as part of the provisions in their will.
"Making provisions for digital inheritance in a will or codicil is relatively straightforward."
