O2 has slammed a UK law firm for issuing letters to web users suspected of illegal file-sharing, saying they "bully or threaten" consumers.

In January, it was revealed that ACS:Law had issued letters to around 150 Brits, claiming they had illegally downloaded content from the web.

ACS:Law said it was acting on behalf of DigiProtect, an anti-piracy firm that represents a number of copyright holders, and the recipient was required to pay a £500 fine and sign a legal undertaking agreeing not to illegally file-share in the future.

O2 said it was "legally obliged" to provide the law firm with contact details for a number of its customers after IP addresses belonging to the ISP were identified as having taken part in illegal file-sharing.

The mobile network said it preferred the "win-win approach of encouraging the development of new business models that offer customers the content they want, how they want it, for a fair price".

However, Andrew Crossley of ACS: Law said the letters did not bully or threaten anyone.

Crossley told the BBC: "We send out letters of claim to account holders of internet connections where those internet connections have been identified as being utilised for illegal file-sharing of our clients' copyrighted works.

"Our letter makes an enquiry in that regard and invites the recipient of our letter to respond to this evidence. In addition they are invited to enter into a compromise to avoid litigation."

Music industry body, the BPI, slammed the tactics used by ACS:Law.

"We don't favour the approach taken by ACS:Law to tackle illegal file-sharing," Adam Liversage from the BPI told the BBC.

"Our view is that legal action is best reserved for the most persistent or serious offenders - rather than widely used as a first response."

But Crossley claimed the BPI was "letting its members down".

"I think they are scared of alienating their customers," he said.

"My clients don't have the same fear. They take the view that the people they target aren't their customers because they are stealing from them," he said.

See also: Legal firm faces investigation over net piracy letters