It is easier for the authorities to find out if you - or your children - have been illegally downloading music than you think. When Virgin Media started writing to some of its customers saying that they had been downloading illegally, it was able to include the date and time of the activity as if it were issuing a speeding ticket.

How does it work? Key to the identification process is the IP address, the unique internet address that identifies an individual connection, and ultimately a person’s house and home. Use a file-sharing service such as Limewire to share or copy music and the IP address can be seen by those also on the system with the visibility of a number plate to those who know how to get it.

But it is not possible to link the number to a person without the cooperation of an internet provider and until now the internet companies wanted to wait for a court order before releasing an alleged infringer’s details.

That was too expensive and time consuming for the record industry to obtain - costing about £1,000 a time. And they realised that it was not smart to sue their customers as the first response to Mary Smith downloading the latest Madonna album. However, today’s government-brokered agreement has the potential to change that as the internet providers have agreed to do the letter writing on behalf of the music and film business.

Letter writing is also a gentle way to warn internet users. The diehards will not care but many parents will worry about what their children are downloading and internet cafés and universities will come under pressure to rethink their policies if they are at war with their technology suppliers.

Most people who have downloaded illegally will not find themselves at risk of sanction - not even the squeeze in their download speed that is threatened - but they will have the realisation that everything they do online leaves a trace.

And it is all done without scanning the actual traffic that passes over the connection, so it is harder to argue that it is intrusive. It is as big a change as the introduction of speed cameras. It won’t stop all illegal downloading, but it will change some people’s behaviour just as the cameras do. Watch out.

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