Up to eight million people are at risk of ID fraud after a hacker breached the security system of the world's largest hotel chain.
An Indian hacker broke into the IT system of Best Western Hotel Group and stole personal details of everyone who has stayed there in the past 12 months.
The details, which included home addresses, phone numbers, place of employment and credit card details, were sold on through an underground network controlled by the Russian Mafia.
The information is thought to be worth up to £2.8billion. Experts say that if it falls into the wrong hands it could spark a 'major crimewave'.
'They've pulled off a masterstroke here,' said security expert Jacques Erasmus, an ex-hacker who now works for the computer security firm Prevx.
He added: 'There are plenty of hacked company databases for Sale
online but the sheer volume and quality of the information that's been stolen in the Best Western raid makes this particularly rare.
'The Russian gangs who specialise in this will have been exploiting the information from the moment it became available. In the wrong hands, there's enough data there to spark a major European crimewave.'
Best Western became aware of the theft on Thursday night.
It instantly disabled the log-in account from which the information was stolen, but not before the details of millions of people had been removed.
Tim Wade, head of marketing for Best Western in Britain, said it was 'unlikely' the thieves got details of every booking in Europe because of the way their system worked.
He added: 'We are investigating further and working with our credit card partners to ensure the interests of our guests are protected.