The UK Government's Online Safety Bill threatens free speech and may be incompatible with human rights law, a top media QC has said.

In a legal opinion for the campaign group Index on Censorship, Gavin Millar QC, of Matrix Chambers, said the bill would lead to algorithms deciding what people can and cannot say online.

Under the proposals, social media companies would be placed under a legal duty to root out "legal but harmful" content considered "likely" to result in "psychological harm".

Clauses in the legislation are ill-defined, leading to fears that content deemed 'offensive' in Silicon Valley, such as orthodox Christian beliefs about marriage or sexual ethics, could be proscribed.

Mr Millar said UK Ministers have failed to recognise that the legislation would interfere with protections on free speech under the European Convention on Human Rights.

Calling for "statutory safeguards" to guard free speech, he said getting the bill wrong could see a: "'Balkanised’ European internet" in which free speech is more contested and restricted in the UK than any other country.

The QC added: "This worrying prospect cannot be good for our democracy".

Sir Trevor Phillips, chairman of Index on Censorship, commeted: “The ‘legal but harmful’ grey area the government has constructed remains huge and will see tech giants be compelled to embark on a censorship-spree of our social media timelines and online news.”

A spokesman for CARE commented:

"We are keenly aware of free speech concerns tied to other aspects of the government's online safety regime. The proposals on 'legal but harmful' content have the potential to curb free expression and religious liberty if Ministers don't get them right.

"Vague speech laws have clear problems. Westminster need only look at the Scottish Government's recent Hate Crime Bill for proof of that. We will be monitoring the UK Government's proposals closely in the months ahead and championing the central importance of free expression."

Read a Q&A on the Online Safety Bill by CARE's James Mildred