The Commission wants the UK to do more to protect privacy
The UK government has been accused of failing to protect citizens' privacy by the European Commission.
It said the government should have done more to guarantee online privacy when trials of a controversial ad-serving system were carried out in 2006.
The Commission said it had now started the second phase of legal action over the trials.
If the UK fails to answer the criticism satisfactorily, it faces being taken to the European court.
"People's privacy and the integrity of their personal data in the digital world is not only an important matter, it is a fundamental right, protected by European law," said Viviane Reding, EU telecoms commissioner.
The Commission started the legal action following trials of the Phorm ad-serving system on BT's network in 2006 and 2007.
Although the UK government was happy with the way that the trial was run, critics said BT's customers who were unwittingly enrolled in it should have had the chance to opt out.
Brussels said this showed that UK laws, particularly The Regulation of Investigatory Powers Act, did not do enough to protect data about the e-mails and web browsing habits of citizens.
"I therefore call on the UK authorities to change their national laws to ensure that British citizens fully benefit from the safeguards set out in EU law concerning confidentiality of electronic communications," said Ms Reding.
The Commission has sent a letter containing its opinion to the UK which now has two months to respond. If the Commission is unsatisfied with the response it could take the case to an EU court and perhaps force a change in UK law.
In response a Home Office spokesperson said: "We are firmly committed to protecting users' privacy and data. We are considering the Commission's letter and will respond in due course."