Tens of thousands of people are feared to have had their computers infected by booby-trapped adverts on websites including the London Stock Exchange as the full extent of a cyber-attack which began on Sunday becomes apparent.
The scam, which also involved ads on Autotrader, Vue and six other websites, began on Sunday after cyber-criminals hacked into an ad firm's IT system.
Malicious adverts were then released which caused fake virus warnings to pop-up on computers belonging to those surfing the affected sites.
The full extent of the cyber-attack, which began on Sunday, has only come to light today
After telling them that their computer was infected, the bogus diagnostic screen asked for payment to remove the 'infection'.
It is thought the scam only affected PC users running Safari, Chrome or Firefox browser.
Some of the websites hosting the malicious ads, including the London Stock Exchange, temporarily closed their sites.
IT managers at the affected advertising company, Unanimis, then spent three hours removing the malware.
'The adverts they chose to modify were not being widely distributed,' David Nelson, operations and IT director at Unanimis told the BBC.
'This, coupled with the attack taking place on a Sunday evening, limited how many people fell victim.
'We have to count ourselves lucky in some respects.'
An investigation is taking place into how the cyber-criminals got access to the Unanimis server in the first place, and why the breach was not detected earlier.
The company today declined to identify all the sites that had fallen victim to the hack but said the advertising network - owned by the Orange France Telecom Group - had informed all those affected.
Patrik Runald, senior research manager at Websense, suggested a lot of people had been targeted by the rogue warnings.
'We believe that quite a large number of sites were showing these adverts,' he told the BBC, suggesting the figure could have reached 'tens of thousands'.
