Up to 13,000 BT customers accused of illegally downloading content are likely to receive legal letters in the new year.

The letters from ACS:Law Solicitors will be demanding payments of hundreds of pounds, and thousands of customers from other internet service providers (ISPs) may also be affected.

ACS Law started sending letters to people it believed to be guilty of using illegal peer-to-peer sites in May this year but other firms, such as Davenport Lyons, have been sending similar letters since 2007.

25,000 letters are estimated to have been sent out by law firms in the past two years, according to data from Being Threatened?, which claims to protect innocent individuals in such cases. Now the scale has reached unprecedented levels and marks a significant step up in copyright holders pursuing illegal downloaders.

ACS:Law obtains its information on individuals who have been file sharing through data monitoring companies that track file-sharing networks.

Andrew Crossley, a lawyer at ACS:Law, said that the mailout to BT customers was the result of information from a German tracking client called DigiProtect, which apparently identified 25,000 IP addresses linked to illegal downloading.

ACS:Law sent the Digiprotect data to BT which, as an ISP, is legally obliged to send back the names and addresses of the customers linked to the IP addresses.

Crossley explained that many of the 25,000 addresses will link back to the same individual, and only around half will receive letters once the redundant and repeat addresses are omitted.

BT said that it could take up to nine months to supply ACS:Law with the details, which would see the letters sent next August, although Crossley expects the details to be handed over before that, probably in January.

ISPs including TalkTalk and BT have complained that music, film, software and video game publishers have failed to prosecute illegal downloaders with sufficient rigour, and that this has led to business secretary Peter Mandelson's three-strikes proposals to tackle illegal downloading, which the ISPs strongly oppose.

Mandelson's proposals will see individuals found downloading illegal content cut off from the internet after multiple attempts and warning letters.

However, ISPs claim that internet access should be considered a human right and should not be restricted without a court order. ISPs are also against the proposals because they will have to bear the brunt of the cost of enforcing them.

The European Telecoms Package was finally passed this week and, although widespread debate remains on how the wording should be interpreted, there is growing evidence that the European Union will not support Mandelson's plans because of the importance of a judicial process.

If this is the case, rights holders will have to continue to rely on companies such as DigiProtect, which is likely to lead to more letters from law firms to customers deemed to have downloaded content illegally.

"This is what we have been saying all along. There are already processes in place to prosecute illegal downloaders. Copyright infringement is wrong and rights holders should enforce their rights and use established procedures to prosecute," said a BT spokesman.

Another area of controversy is the amount of money being made by the tracking companies and law firms. ACS:Law is currently trying to introduce minimum fines of £750 in a drive to introduce the notion of statutory damages for copyright infringement, as is in place in the USA. The firm is also warning letter recipients of the cost of going to court.

There are also organisations which argue that many recipients of the letters are likely to have had their IP address hijacked by those wanting to access illegal content without being caught.

Being Threatened? has argued that innocent people often pay to get rid of the problem, and described the large amount of stress it can cause.