The letter outlined a campaign privacy advocate's plan to wage a battle against Internet advertiser DoubleClick, which by the late third quarter plans to acquire market researcher Abacus Direct in a stock-swap deal.
DoubleClick's ability to track people's online habits combined with Abacus's database of consumer information would result in an unprecedented surveillance of Net users, critics say.
"We write to urge you to abandon the proposed merger of your companies on the grounds that it would severely undermine the privacy of Internet users," the letter states.
As previously reported, the critics also plan to file a complaint with federal regulators to try to block the merger.
"We don't know whether the merger approval will go through the Justice Department or the Federal Trade Commission, so we are still deciding on the legal basis of the complaint," said Jason Catlett, founder of Junkbusters, a clearinghouse for privacy-protection measures.
The merger will allow DoubleClick's network of 1,300 Web sites to exchange people's personal data with Abacus's collection of 1,100 catalog companies, although both companies allow consumers to opt out of their databases and both have disclosure policies about what they do with data once it is collected.
But in their letter, privacy advocates said people need more choice regarding their data.
"The only fair approach would entail fully informed affirmative consent [opt-in]," the letter states.
Privacy advocates also want assurances about DoubleClick's DART system, which delivers targeted ads to consumers by using so-called cookies to track surfing habits.
The advocates' letter states, "If you are willing to issue a guarantee that the merged entity and successors will never associate a name and address with a cookie without the affirmative informed consent of the individual concerned, please state this in public."
Aside from complaining to regulators, consumers groups also plan to discourage mutual funds from investing in the companies and will appeal to the companies' shareholders to vote down the merger.
News.com's Courtney Macavinta contributed to this report.