The UK’s six largest ISPs will today announce a plan to tackle illegal music and movie downloading by sending warning letters to thousands of customers.

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Threatened with legislation by April if they fail to implement their own solution, two ISPs, Virgin Media and BT, already began sending letters last month to those suspected of downloading music; users of services like LimeWire identified by the British Phonographic Industry. Now Orange, Tiscali, BSkyB (NYSE: BSY) and even Carphone Warehouse are set to sign up to an extended scheme that also covers movies via the Motion Pictures Association of America, morning papers reported. The BBC says it’s a memorandum of understanding between the ISPs and the government‘s Department for Business, Enterprise & Regulatory Reform (BERR).

FT.com: “The service providers have agreed in principle to a code of practice for dealing with persistent offenders, setting out agreed sanctions. The voluntary code would be overseen by Ofcom, the broadcasting regulator.” Rather arbitrarily, the story says ISPs will write to 1,000 offenders per week; it’s a three-month trial. BPI says “hundreds of thousands” of letters will be sent in the first year.

Sixty-three percent of people download music from P2P networks - an average of 53 illegal tracks per month, according to June University of Hertfordshire research for British Music Rights. But one warning from an ISP would be enough to stop 70 percent of illegal file sharers in their tracks, according to Entertainment Media Research in March.