Several of the world's biggest internet companies have blasted plans to force ISPs to block access to sites carrying pirated music and video.

Google, eBay and BT are among a list of signatories on a letter published today in the Financial Times, which calls the proposed law a threat to freedom of speech and burdened with "obvious shortcomings".

The last-minute amendment to the Digital Economy Bill won support in the House of Lords last week. It will give courts the power to force ISPs to bar access to sites accused of copyright infringement.

The letter, which is also signed by several of Britain's biggest broadband providers, says that "endorsing a policy that would encourage the blocking of websites by UK broadband providers or other internet companies is a very serious step for the UK to take".

"There are myriad legal, technical and practical issues to reconcile before this can be considered a proportionate and necessary public policy option," it adds.

The signatories claim the amendment was tabled too late in the bill's passage through Parliament for the legal implications to be fully considered. The internet firms also claim that the cost of defending court orders to ban websites will simply lead to ISPs acquiescing to rights holders' demands.

Consumer groups, tech-savvy celebrity Stephen Fry and Labour MP Tom Watson have also added their names to the letter.