A digital black hole will open in Britain's national memory without a change in the law to ensure the capture and recording of UK websites, the head of the British Library has warned.

After six years struggling to establish the UK Web Archive, launched today, Dame Lynne Brindley admitted efforts were failing. Her library, working with the National Library of Wales and the Wellcome science library, will by next year have archived just 6,000 of an estimated 8m sites, or 1% of the total.

The National Library of Scotland is also archiving sites. With the average lifespan of a site between 44 and 75 days, and one in 10 lost or replaced each six months, most of the unrecorded 99% are gone for ever.

Brindley's problem is the Legal Deposit Libraries Act, which provides that every UK print publication is automatically deposited by the publishers in the British Library and other copyright libraries, and which was only extended in 2003 to cover online material.

Under the current interpretation of the law, the libraries have to identify and then seek the permission of each individual site webmaster before adding a site to the archive.

A spokesman for the library described the situation as ridiculous, and said: "We've got the know-how but we need the rules to say we don't need to ask permission. We're archiving for the nation rather than commercial gain."

The Department for Culture, Media and Sport is consulting on how well the act is working.

The UK Web Archive would not contemplate collecting every site, but is targeting those considered to have genuine research interest. Sites already in the bag include One&Other, Antony Gormley's arts project last year that put 2,400 people chosen by ballot on to the fourth plinth in Trafalgar Square for an hour each.

Vanished companies such as Woolworths, whose stores are now boarded-up shells on many high streets, live on in the archive. There are also plans to immortalise the websites of the scores of MPs planning to retire at the general election.