A bar on pirate websites would be a direct threat to freedom of speech, search companies and activists have warned.
The claim comes after the Tories and Liberal Democrats forced an amendment to the Digital Economy Bill giving courts the power to make internet service providers block certain websites.
Google, Yahoo, Facebook and eBay, along with technophile television host Stephen Fry, have signed a letter objecting to the potential change in the law.
Speaking in a letter to the Financial Times, the 16 signatories criticised the proposals as rife with "obvious shortcomings", adding that they would not tackle the problem.
"There are myriad legal, technical and practical issues to reconcile before this can be considered a proportionate and necessary public policy option," the letter said.
Broadband providers Orange, BT, Virgin and Carphone Warehouse also criticised the amendment.