Every book is automatically deposited in the British Library
The UK's online heritage could be lost forever if the government does not grant a "right to archive", a group of leading libraries has said.
The British Library, along with other institutions, has been archiving UK websites since 2004 but has only been able to cover 6,000 of an estimated 8m.
Currently, it must ask permission from website owners before archiving them.
The group, which has just made its UK Web Archive available to the public, warned of "a digital black hole".
"We've got the know-how but we need the rules to say we don't need to ask permission," said a spokesman for the British Library.
"We're archiving for the nation rather than commercial gain."
The British Library believes the UK Web Archive could prove as useful to historians as ancient pamphlets and other ephemeral material in its archive.
The consortium, which also includes the National Library of Wales and the Wellcome Library, is lobbying the government to clarify elements of the Legal Deposit Libraries Act.
The act, which among other things means that every UK print publication is automatically deposited by publishers in the British Library, was extended in 2003 to cover online material.
But the British Library says it never clarified what steps had to be taken before electronic material was recorded.
"We're in the ridiculous position where we have to ask permission of each webmaster before we archive a site," the spokesman said.
The Department of Culture Media and Sport is currently consulting on the act.
"Limited by the existing legal position, at the current rate it will be feasible to collect just 1% of all free UK websites by 2011," said British Library chief executive, Dame Lynne Brindley.
She said the UK Web Archive project was necessary to help "avoid the creation of a 'digital black hole' in the nation's memory".
The project was first launched in 2004 and so far has archived web forums where people discussed the London bombings in 2005, material documenting the credit crunch - including the sites of companies which went bust, like Woolworths - and defunct political sites, such as those of MPs who have died or lost their seats.
"We can't make a judgement about what people in the future will find useful," said the British Library spokesman.
"The most ephemeral material from the past contains the most social detail - such as graphic design, and literacy. Websites are the successor to that."
The British Library said research showed that the average life expectancy of a website was just 44 to 75 days, and suggested that at least 10% of all UK websites were either lost or replaced by new material every six months.
Several other countries - including Australia, New Zealand, Finland and Portugal - have already started to archive their national web material.
Other informal projects - such as the WayBack Machine - also exist.