An open letter has been published to seven of the UK's biggest online businesses to request that they do not take up new ad-serving system, Phorm.
The letter, written by the Open Rights Group, has been sent to the directors of Facebook, Google, Yahoo, AOL, Ebay, Amazon and Microsoft.
"You may already be aware of the very significant concerns being expressed by many of your UK internet customers about the interception and processing of their data whenever it is viewed by customers whose ISPs deploy the Phorm/Webwise system," the letter said.
Around 21,000 people have signed a petition to ask for Phorm to be the subject of an investigation into whether or not it breaks Europe-wide laws on privacy.
Virgin Media, BT and TalkTalk have all expressed an interest in using Phorm in the past.
The Information Commissioner gave permission for Phorm to go-ahead last year, after ruling that they had not breached guidelines.
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