Historians face a "black hole" of lost material unless urgent action is taken to preserve websites and other digital records, the head of the British Library has warned.

Just as families store digital photos on computers which might never be passed on to their descendants, so Britain's cultural heritage is at risk as the internet evolves and technologies become obsolete, says Lynne Brindley, the library's chief executive.

Writing in today's Observer, Brindley cites two examples of losses overseas. When Barack Obama was inaugurated as US president last week, all traces of George Bush disappeared from the White House website, including a booklet entitled 100 Things Americans May Not Know About the Bush Administration, which is no longer accessible.

There were more than 150 websites relating to the 2000 Olympics in Sydney, she continues, but these, too, vanished instantly at the end of the games and are now stored only by the National Library of Australia. "If websites continue to disappear in the same way as those on President Bush and the Sydney Olympics - perhaps exacerbated by the current economic climate that is killing companies - the memory of the nation disappears too," Brindley writes. "Historians of the future, citizens of the future, will find a black hole in the knowledge base of the 21st century."

Historians have become increasingly concerned that while the Domesday Book, written on sheepskin in 1086, is still easily accessible, the software for many decade-old computer files - including thousands of government records - already renders them unreadable. The ephemera of emails, text messages and online video add to the headache of the 21st-century archivist.

"Too many of us suffer from a condition that is going to leave our grandchildren bereft," Brindley states. "I call it personal digital disorder. Think of those thousands of digital photographs that lie hidden on our computers. Few store them, so those who come after us will not be able to look at them. It's tragic."

She believes similar gaps could appear in the national memory, pointing out that, contrary to popular assumption, internet companies such as Google are not collecting and archiving material of this type. It is left instead to the libraries and archives which have been gathering books, periodicals, newspapers and recordings for centuries. With an interim report from communications minister Lord Carter on the future of digital Britain imminent, Brindley makes the case for the British Library as the repository that will ensure emails and websites are preserved as reliably as manuscripts and books. "This vision of a digital Britain must include the critical public service of preserving digital Britain's collective memory and digitising the unrivalled content within the British Library."

The library plans to create a comprehensive archive of such "notoriously ephemeral" material from the UK web domain - there are about eight million .uk domain websites, growing at a rate of 15-20% every year. It also has a collecting and archiving project for the London 2012 Olympics.

In 2007 the library worked with Microsoft and the National Archives at Kew to prevent a "digital dark age" by unlocking millions of unreadable stored computer files. Microsoft installed the Virtual PC 2007, allowing users to run multiple operating systems simultaneously on the same computer and unlock what are called "legacy" Microsoft Office formats dating back 15 years or more.

The library and national archives have set up projects to capture daily exchanges of information almost entirely now transmitted by emails and texts. Government departments are storing emails and archiving them at Kew, and the library is encouraging individuals to store theirs voluntarily.

Historians regard some of today's electronic data as a vital legacy that must be protected. Tristram Hunt, of Queen Mary College, London University, said: "It's essential that mainstream institutions such as the National Gallery or the White House or the Ministry of Defence keep email correspondence, and I think they're quite good about that now. We saw all that come out in the Hutton report and it was fascinating. That's an absolutely essential historical record."

But Hunt argued that libraries and other institutions need to be selective. "On the other hand, we're producing much more information these days than we used to, and not all of it is necessary. Do we want to keep the Twitter account of Stephen Fry or some of the marginalia around the edges of the Sydney Olympics? I don't think we necessarily do."

There is already one stark warning from history. The BBC's Doomsday Project of 1986, intended to record the state of the nation for posterity, was recorded on two 12inch videodisks. By 2000 it was obsolete, and was rescued only thanks to a specialist team working with a sole surviving laser disk player.

• This correction was published in the Observer's For the record column, Sunday 1 February 2009. Above we mentioned a "sole surviving laser disk player" which was used to rescue the BBC Domesday Project of 1986 as the 12-inch videodisks on which it is encoded could not be read on obsolete equipment. We are happy to report that a few enthusiasts have told us that they are still in possession of working Domesday hardware systems.