The Department of Health has spent more than £2.5 million advertising on Google in the past year, it emerged.
Using the internet search engine's pay-per-click "AdWord" service, the department spent £2,720,457.11 between February 1, 2009 and January 31 this year, Health Minister Phil Hope said.
It had paid for 21,939 "active search terms", he revealed, but said setting out what had been chosen could put the department at a "future competitive disadvantage".
Google AdWord allows clients to select relevant search terms which will bring up an advert for their services. The clients are then charged if internet users click on the advert.
In a written Commons answer to Tory Nick Hurd (Ruislip Northwood), Mr Hope said the department's active terms included searches on specific campaigns and NHS Choices - the "front door" to the NHS, according to its website.
"In relation to which Google keywords have been bought for use, such information is commercially sensitive," Mr Hope said.
"The commercially competitive nature of the cost of Google AdWord keywords means that putting specific information in the public domain on actual keywords used could put the department at a future competitive disadvantage."
Campaign group the Taxpayer's Alliance hit out at the spending.
Campaign director Mark Wallace said: "The Government's massive advertising budget has become utterly excessive.
"Whether it's online, on the TV or on the radio, you can barely move for taxpayer-funded adverts telling people how to live their own lives."