The politicians sent letters to 15 companies last week, each asking 15 questions relating to online data-gathering procedures and policy.

Markey and Barton are co-chairmen of the House of Representatives privacy caucus, and say they were prompted in to action by a recent series of articles in the Wall Street Journal investigating how website owners collect and use visitor data, largely for marketing purposes.

Referencing the initial report, published on 30 July, Markey and Barton said: “We are troubled by the findings in this report, which suggest that the price of consumers’ daily use of the internet increasingly is surrender of their personal information, including web search terms, data on websites visited, content and application preferences, and other details.”

The congressman have asked for responses to their questions by 12 August.

Both the House and Senate are considering privacy legislation, while the Federal Trade Commission said recently it was still mulling the possibility of implementing some form of do-not-track list to provide consumers with a universal opt-out for tracking.

Legislative efforts appear primarily concerned with the use of web tracking technologies to build up detailed profiles of individuals for targeting purposes, rather than data that is collected for market research purposes, which is usually presented in anonymous and aggregated form.

Yet both marketers and researchers use similar technology to achieve their goals, and the fear is that research too might find itself subject to the same privacy laws as marketing. The Marketing Research Association has already warned that one draft privacy bill “impacts far more than advertising and commercial enterprises”.