Google will today expand its offering to advertisers by unveiling a service which lets them track which websites their target audiences are visiting.

A new tool called AdPlanner is expected to offer advertisers a range of statistics - including demographic details - about particular websites, which in turn could be used to target advertising more effectively, it was reported.

The service, which is due to be announced this evening, signals Google's intention to ramp up its advertising business, which to date has focused on placing adverts next to search queries conducted on the web.

AdPlanner would effectively couple Google's existing advertising business with so-called "web analytics" tools, which measure audience behaviour on the web.

According to reports in the US, Google is planning to gather information about websites from a range of sources, including its own search engine but also third parties and, possibly, more traditional panels which track consumer behaviour online.

This data would then be offered to advertisers at the time they buy adverts through DoubleClick, the online advertising platform Google owns.

A Google spokesman declined to comment.

AdPlanner, it is hoped, will help advertisers make better judgments about the types of sites on which they buy adverts, based on concrete data such as the age, gender and previous online behaviour of people who visit such sites.

It is understood that any data offered to advertisers as part of the service would be anonymous.

One Google source suggested that there would be some overlap between the new service and another recent offering from Google called 'Trends for websites', which allows anyone to measure websites' audiences using a combination of Google search data and other information that web users have opted to share about their behaviour.

Marketers are increasingly calling for more detailed information about the way consumers use the web, and Google's new tool is thought to be similar to another service called Quantcast, which allows websites to provide highly specific - though anonymous - information about their audiences to advertisers.

AdPlanner, which is expected to be announced at an advertising industry event this evening, is likely to provide a jolt to the web analytics industry, which is dominated by companies such as comScore and Nielsen Online.

Such companies already use consumer panels to gather information about how people use the internet, which they then sell on to advertisers, but Google's vast reach may enable it to provide yet more detailed information to advertisers. In addition, AdPlanner is expected to be free - at least to begin with.

Neither comScore nor Nielsen Online was immediately available for comment.

Separately, Google is expected this week to roll out another tool designed to show how people browsing the web respond to online advertising. By comparing the behaviour of people who are exposed to particular adverts with those who are not, the company hopes to give advertisers feedback about which campaigns are are successful.

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