Amendments made to the digital economy bill by the House of Lords threaten freedom of speech and will lead to British websites being blocked without due judicial process, the chief executives of leading technology companies said today.

The heads of the four largest UK internet service providers – BT, Orange, Virgin Media and TalkTalk – as well as Google, Facebook, eBay and Yahoo have all co-signed the letter, along with consumer groups, academics and the technophile television host Stephen Fry, objecting to amendment 120A to the bill, which was added to the bill last week with support from Liberal Democrat and Conservative peers.

Ministers had been seeking powers to amend copyright law and impose conditions or fees where infringements were taking place.

But the Conservatives and Liberal Democrats succeeded in removing the measures from the bill last week, replacing them with a more specific amendment handing courts the power to force internet service providers (ISPs) to block certain websites.

In a letter to the Financial Times , the online giants argue that the rules, if they become law, would fail to tackle copyright infringement as intended. The amendment has "obvious shortcomings", the 16 signatories say.

The letter says: "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."

The amendment had been roundly criticised last week when it was added, as critics pointed out that it could be used to block sites such as YouTube.

But Lord Tim Clement-Jones, one of the backers of the amendment, said last week that the intention was to deal with "cyberlockers" – a system that allows individuals to swap large files directly, rather than sending them by email or storing them on websites.

The House of Lords passed the amendment last week, replacing a clause that would have given broad powers to ministers to change the Copyright Act to respond to new forms of online infringement without the need for primary legislation.

But the letter's signatories called the amendment "bitterly disappointing", and explained: "Put simply, 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. To rush through such a controversial proposal at the tail end of a parliament, without any kind of consultation with consumers or industry, is very poor law-making."

Responding to the letter, the chief executive of UK music industry body the BPI, Geoff Taylor, said that the amendment provided a "clear and sensible" way of dealing with illegal downloading.

Taylor added that the signatories to the FT's letter have acknowledged that illegal downloading has to be dealt with.

"The amendment adopted by the House of Lords provides a clear and sensible mechanism to deal with illegal websites," he said.

"Contrary to the claims in the letter, service providers would in every case be able to ensure that the decision as to whether a site should be blocked is made by the high court. The court would be required to consider the extent of legal content on a website, any impact on human rights, and whether the website removes infringing content when requested. So the suggestion that the clause would lead to widespread disruption to the internet or threaten freedom of speech is pure scaremongering.

"The signatories to the letter recognise that dealing with illegal websites is a legitimate concern, and have argued in the past that action against illegal downloading should focus on commercial operators. Removing unfair competition from clearly illegal websites will encourage investment in legal online services and improve the legal internet experience for everyone."

The digital economy bill is expected to be pushed through before parliament is dissolved for the general election, widely expected to happen on 6 May. If it reaches a second reading by early April, when an election would be called, it could go into the "wash up" – the process at the end of a parliament when bills that have not been passed are hurried through. The government would need cooperation from the opposition to achieve that with the bill – but it is not clear whether the Tories, who have objected to elements of the bill, as the "landline tax" of £6 a year to help pay for next-generation broadband, would support it.

Lord Clement-Jones had said the provisions, approved by 165 votes to 140, would protect the creative industries by preventing access to websites where films and music were being provided illegally.

He told peers: "I believe this is going to send a powerful message to our creative industries that we value what they do, that we want to protect what they do, that we do not believe in censoring the internet but we are responding to genuine concerns from the creative industries about providing a process whereby their material can be satisfactorily accessed legally."

Lord Clement-Jones said the "blanket nature" of the government's original intention was "objectionable". He argued the new proposals were "more proportionate, specific and appropriate".

The bill extends the role of media regulator Ofcom to include communications infrastructure and media content, and to appoint providers of local news in ITV regions.

It also includes powers to stop under-age children getting hold of violent computer games and contains measures to help the switchover to digital radio.