Britain’s leading internet thinktank has raised questions over the legality of the technology used by Phorm, one of a growing number of behavioural advertising companies.

The Foundation for Information Policy Research (FIPR), which monitors the impact of IT on privacy, has sent an open letter to the Information Commissioner setting out its concerns.

Phorm’s platform tracks the behaviour of internet users to generate adverts tailored to their interests on websites signed up to its technology.

Communications firms including BT, TalkTalk and Virgin have signed up to the technology to make their online advertising more relevant.

In its letter, FIPR argues that Phorm contravenes the Regulation of Investigatory Powers Act by intercepting information.

FIPR treasurer Richard Clayton said: “The Phorm system is highly intrusive. It’s like the Post Office opening all my letters to see what I’m interested in, merely so that I can be sent a better class of junk mail.”

But Phorm’s chief executive Kent Ertugrul said Phorm was “very, very comfortable” it was not breaching any UK laws.

A Phorm spokeswoman said searches and browsing behaviour could not be traced back to an individual and that Phorm did not store search details. “Once the technology makes a conclusion, the data is discarded,” she said.

Late last year Google announced its £1.5bn purchase of advertising targeting company DoubleClick, increasing fears about its retention of user search data, which is only anonymised after 18 to 24 months.