Internet giant Google has drawn up plans to compile psychological profiles of millions of web users by covertly monitoring the way they play online games.

The company thinks it can glean information about an individual's preferences and personality type by tracking their online behaviour, which could then be sold to advertisers. Details such as whether a person is more likely to be aggressive, hostile or dishonest could be obtained and stored for future use, it says.

The move is intended to customise adverts shown to players of online video games by tailoring them to specific tastes and interests. But it has worried privacy campaigners, who said the implications of compiling and storing such detailed information were "alarming".

Sue Charman of online campaign Open Rights Group said: "I can understand why they are interested in this, but I would be deeply disturbed by a company holding a psychological profile.

"Whenever you have large amounts of information it becomes attractive to people - we've already seen the American federal government going to court over data from companies including Google."

The plans are detailed in a patent filed by Google in Europe and the US last month. It says people playing online role playing games such as Second Life and World of Warcraft would be particularly good to target, because they interact with other players and make decisions that probably reflect their behaviour in real life.

The patent says: "User dialogue (eg from role playing games, simulation games, etc) may be used to characterise the user (eg literate, profane, blunt or polite, quiet etc). Also, user play may be used to characterise the user (eg cautious, risk-taker, aggressive, non-confrontational, stealthy, honest, cooperative, uncooperative, etc)."

The information could be used to make adverts that appear inside the game more "relevant to the user", Google says.

Players who spend a lot of time exploring "may be interested in vacations, so the system may show ads for vacations". And those who spend more time talking to other characters will see adverts for mobile phones.

The patent says Google could also monitor people playing on any game console that hooks up to the internet, including the Sony PlayStation, Nintendo Wii and Microsoft's Xbox. It says information could be retrieved from previous game details saved on memory cards: "Such saved information may be thought of state information, and offers a valuable source of information to the advertisers."

Not all the inferences made by monitoring user activity rely on subtle psychological clues, however. "In a car racing game, after a user crashes his Honda Civic, an announcer could be used to advertise by saying 'if he had a Hummer, he would have gotten the better of that altercation', etc," the patent says. And: "If the user has been playing for over two hours continuously, the system may display ads for Pizza Hut, Coke, coffee."

Some web companies already scan online activity such as internet searches and email for keywords in order to display adverts based on basic attributes and behaviours. But the extensive profiles proposed in the patent go some way beyond existing examples. It would be relatively straightforward to combine information sources to put names and addresses to the anonymous profiles, experts said.

When contacted by the Guardian, Google said it did not have any plans to roll out the technology in the near future, and that it was just one of a large number of patents that it has filed in recent months. A spokesman said: "Google registers different patents irrespective of whether we actually intend to use them."

The company, which has built its success on delivering adverts over the internet, spent $23m (£11.5m) this year acquiring a game advertising company called AdScape. The move was seen as part of a broad expansion of its business into other media, including television and radio.

Although using the proposed profiling techniques would require games publishers to actively incorporate Google's technology, industry experts said that games publishers are increasingly looking for new ways to make money.

"Publishers are becoming more like media companies," said Justin Townsend, the chief executive of IGA, an in-game advertising company. "More games are being optimised for advertising." But he said that privacy concerns should be paramount. "Both consumers, publishers and the advertising industry are very aware of privacy issues," he said. "You cannot afford to be vague in these areas."

Booming market

Experts estimate that around $500m (£250m) will be spent on advertising inside games this year, rising to $2bn by 2010. While more straightforward games such as online poker are already hugely profitable, it is predicted that virtual three-dimensional worlds and role-playing games will soon be among the biggest moneyspinners on the net.

One of the biggest titles is the Lord of the Rings-style fantasy game World of Warcraft, which has more than 8 million subscribers worldwide who each pay around £9 per month to take part in the action.

Second Life, an online virtual community which mimics the real world, has more than 5 million registered users and 1.2 million regular visitors. It is famous for allowing its residents to buy and sell virtual goods, or rent "land" inside the game for up to $300 per month.

A recent report by analysts Screen Digest suggests that the market for virtual worlds in the west already surpasses $1bn per year.