BT and Carphone Warehouse will resist Lord Mandelson's plans to disconnect persistent illegal file sharers.
Charles Dunstone, chief executive of Carphone Warehouse, said that his company TalkTalk would refuse to slow down or cut off access unless ordered to by the courts.
He said in a statement: "What is being proposed is wrong in principle and it won't work in practice."
BT said that tampering with connections would result in millions of broadband consumers facing rising costs and poorer internet experiences.
Both companies believed that it would increase broadband costs, infringe the human rights of innocent users, and drive offenders to find other ways of downloading illegally.
Dunstone said: "The unintended consequence of Mandelson's plan will be to encourage more Wi-Fi and PC hi-jacking and expose more innocent people to being penalised wrongfully."
BT did not repeat Dunstone's views on the legal measures, but it did say that costs would rise and add 2 a month on the cost of home subscriptions.
