TUI Travel-owned user review site Holidays Uncovered suffered a security breach last week, impacting on how the site appeared in search engines.
Officials are still unsure as to the extent of the problem but have confirmed the attack took place and are investigating the circumstances.
The issue first came to light late on Thursday 16 September when searches on Google revealed the following warning message against each result featuring the user review site.
"This site may harm your computer"
The message is understood to have remained against search results for around ten hours before the attack was halted and measures put in place to prevent it happening again.
TUI Travel director of ecommerce Alun Williams confirmed on Friday that the incident had occurred, possibly through a breach of the site's OpenX advertising system.
The breach was fixed immediately but a review is underway to establish the circumstances, he said.
TUI package holiday brand Thomson bought HolidaysUncovered in September 2007 for an undisclosed fee. The deal triggered a round of similar deals with TripAdvisor snapping up HolidayWatchdog six months later.