Thousands of parents will learn of their children's illegal downloading habits when warning letters arrive at their homes in a battle against internet piracy.

A government-backed drive is targeting the worst offenders of the estimated 6.5 million web users involved in illicit file-sharing of music and films.

Britain's six largest internet service providers, or ISPs, have joined the scheme, amid estimates that the practice could cost the music industry £1bn in the next five years.

One sanction being...