The Carphone Warehouse may face an injunction from the music industry unless it backs down on its plans to ignore illegal file sharers.
It was reported last week that the mobile phone company, which also runs ISP Talk Talk, had rejected plans to implement a 'three strikes and your out' scheme, which would see ISPs tracking and punishing illegal file sharers.
However, The Telegraph reported that Geoff Taylor, chief executive of music industry trade body the BPI threatened to take the ISP to court unless it cut off customers who downloaded illegally, in a letter which was faxed to Talk Talk chief executive Charles Dunstone.
"Accordingly, unless we receive your agreement in writing that within 14 days Carphone Warehouse will implement procedures set out above, we reserve our right to apply to court for injunctions and other relief without further notice to protect our members' rights," read Taylor's fax.
Dunstone told the Telegraph he was surprised by the fax.
"They have been quite heavy-handed in the way they have threatened us. They are trying to position this as 'we are friendly and we all want to work together', but they are threatening legal action at the same time in their first letter to me," he added.
A spokesman for the BPI said: "TalkTalk have more than just a social responsibility to help customers avoid using their accounts illegally. If, as it would appear, TalkTalk are not interested in partnership, we must consider how to proceed."