The envelope containing a letter warning subscribers that their account was being used for illegal file-sharing was printed with the words "Important. If you don't read this, your broadband could be disconnected".
A Virgin Media spokeswoman told OUT-LAW that the message was a mistake.
"We are not accusing our customers of doing anything, we are alerting them to the fact that illegal file sharing has been tracked to their account. This could have been someone else in the house or an unsecured wireless network. This is an education campaign," she said.
The company used information provided by music rights holders' group the BPI to identify accounts which may have been used for copyright-infringing file sharing. The spokeswoman said, though, that no names or addresses were passed to the BPI and that it had been responsible for the envelope, a mistake that it was "rectifying immediately".
France is implementing a policy of forcing ISPs to disconnect copyright infringing users after three warnings and the UK Government has expressed its intention to legislate along similar lines if ISPs and content owners cannot agree a way of clamping down on infringement.
Virgin is the UK's third biggest ISP and the first to co-operate with the BPI on the sending of letters to people whose accounts have allegedly been used for illegal file-sharing.
Virgin claims that the letters are merely intended to educate subscribers about alleged use of their accounts so that they can find out what the source of illegal file-sharing might be.
The BPI, though, has said that it wants ISPs to begin a campaign of suspensions. Chief executive Geoff Taylor said in a statement last month that the BPI wanted ISPs to act.
"We want all ISPs to implement a simple, non-technological solution which involves no spying on their customers or invasions of privacy. We call it three steps," he said. "We collect and pass on to the ISP publicly available information about their customers' illegal filesharing, and ask them to send advisory letters as outlined above. The possibility of account suspension, and the ultimate sanction of contract cancellation, should follow for those customers who choose not to take the advice."
Content producing companies and producers' representative bodies have long argued that ISPs make money from piracy by charging fees for internet access that is then used to pirate their content.
ISPs in turn have argued that they have no role in what a user does with an internet connection just as a postal service has no role in monitoring or governing what a letter or package contains.
Editor's note, 07/07/2008: This story has been amended. It previously suggested that Virgin Media had shared information with the BPI. Virgin Media pointed out that they only receive information and don't give it out. We apologise for the error.