Almost 3,000 households will receive letters from porn makers Ben Dover Productions demanding payment over claims of copyright infringement.
Parent company Golden Eye, acting on behalf of Ben Dover, will target 2,845 people for payment.
The letters will be sent out over the next few months from August.
Only O2 customers will be targeted initially, after a High Court ruling extended only to one ISP. Originally up to 9,000 customers were to be sent letters demanding payment, but this was reduced and the wording of the letters to be sent was also softened.
Golden Eye initially sought £700 from each user, but instead the money owed will be determined for each individual case.
The letters that will be sent can be read here.
It includes information about how accused users can contact the Citizens Advice bureau, and gives each user 28 days to reply in writing.
Golden Eye said it was considering targeting other ISPs in future.
Consumer Focus, which is working with Citizens Advice to help those who will receive the letters said that innocent people should not be unduly pressured to pay when or if they had done nothing wrong.
Mike O’Connor, chief executive of Consumer Focus said:
"We do not condone copyright infringement, but innocent people should not feel bullied into settling claims and they must be made aware of where to go for help.
"We intervened in this case to make sure that consumers are treated fairly. People will not have ready access to the sort of specialised legal advice necessary to respond to allegations of copyright infringement."