Internet service providers and telecoms companies have expressed reservations about European plans to require communications firms to store internet and telecoms data for up to a year.
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European justice ministers agreed in principle to adopt data retention legislation last week, following lobbying by the UK government in the wake of the London and Madrid bombings.
The proposals are designed to create a "level playing field" across Europe, requiring each member state to keep traffic records of phone calls, e-mails and websites visited.
But the London Internet Exchange (Linx), the world's largest internet exchange point, said it was concerned that the plans appeared to go further than the voluntary data-retention system already in operation in the UK.
"We would like something that is possible to implement, is effective and gives police what they need without being unduly costly or obtrusive," said Malcolm Hutty, Linx regulation officer.
"The latest proposal I have seen has retention periods that are longer than we have today. It covers a wider range of data and is not really practical," he said.
ETNO, a lobby group representing large European telecoms firms, said that the proposals could cost a large telecoms company £100m to create a database, and that searching would take too long to be useful for police.