Figures from Google revealed that the site inadvertently hosted an estimated 363 potentially malicious ads during the last 90 days.
An advert provided by a third party-supplier, which has subsequently been removed, was thought to be to blame - clicking on the graphic could lead to dangerous software being downloaded and installed on a userâs computer.
Web browsers have been warning users to avoid the LSEâs site as it may harm their computers.
Paul Mutton, an IT security expert, told the BBC he could not reach the site using Firefox but Google Chrome allowed him access.
"It seemed to work with Chrome but then a few seconds later, without having to click on anything, pop-ups started to appear. I visited the site and it compromised my machine,â he explained.
News of the attack follows a four hour outage on the LSEâs cash equities trading system towards the end of last week.
Xavier Rolet, chief executive officer of the LSE Group, said on Friday: "We sincerely regret the inconvenience that today's disruption to trading has caused our customers.â