Website owners have been warned to keep their registration up to date because lapsed domain names are being snapped up within 10 seconds of becoming available.

Re-registration, which has to be done every two years, can cost as little as £5 but you can pay ten times as much to get an address back - and sometimes far more. Names are regularly sold for more than £100,000, and some for as much as £1m.

Registrants are warned by email to renew, but many do inform registrars that their addresses have changed. There is a 60-day grace period after the expiry date, which the name will no longer work but can still be renewed; but then the name is up for grabs.

A report from Nominet, the not-for-profit company that controls .co.uk registration, says there is a growing secondary market in domain names with some organisations hoarding large numbers of them.

Ownership is often split across a number of companies, so figures are hard to come by, but analysis has shown that around 50 individuals or organisations each own more than 3,000 domain names - between them accounting for five per cent of the .co.uk total.

Trading in domain names is one of two ways "domain warehousers" make money. The other, often done in parallel, is to attach the name to a rudimentary site with commercial links that can earn click-through revenues. Such sites get hits either by clever search-engine optimisation of by having names close to those of legitimate sites.

Phil Kingsland, marketing director of Nominet, said most lapsed names stem from people who have either ceased trading or never really used the addresses.

And he points out that warehousers do not have a completely free rein.

"There is a grey area around using names that people or organisations believe they have a right to," he said.

Nominet has a dispute resolution system for companies who feel their brand is being 'abused' in a web address, though cases do sometimes reach the courts.