Communications executives say proposed amendments to the Digital Economy Bill threaten free speech.

A letter published by the Financial Times says the House of Lords amendment, 120A, would have unintended consequences that far outweigh any benefits it could bring.

The bill would change the UK's procedures on injunctions so that internet service providers (ISPs) would block more websites accused of illegally hosting copyrighted material without a court order, the letter said.

It was signed by the CEOs of BT, Orange, Google, Yahoo, Facebook, eBay, Virgin Media, Carphone Warehouse, the Internet Service Providers Association, Consumer Focus and Open Rights Group, as well as celebrity Stephen Fry, MP Tom Watson and university lecturers.

The letter comes a week after the proposed change attracted hostile criticism from media lawyers and industry observers.

The amendment went beyond even what the controversial US Digital Millennium Copyright Act (DMCA) required, said online copyright lawyer Francis Davey. The US is trying to get DMCA language included in the controversial Anti-Counterfeiting Trade Agreement. ISPs would be better off under DMCA than under Amendment 120a, Davey said.

The letter said, "Blocking access as envisaged by this clause would both widely disrupt the internet in the UK and elsewhere and threaten freedom of speech and the open internet, without reducing copyright infringement as intended."

The writers said there were "myriad" legal, technical and practical issues to reconcile before the amendment could be considered proportionate and necessary.

"In some cases, these may never be reconciled. These issues have not even been considered in this case," they said.

The letter warned that the amendment could harm the UK as a place to do online business and conflict with the broader objectives of Digital Britain.

"This debate has created tension between specific interest groups and the bigger prize of promoting a policy framework that supports our digital economy and appropriately balances rights and responsibilities. All parties should take steps to safeguard this prize and place it at the heart of public policy in this area," the letter said.