Labour proposals for an official definition of 'anti-Muslim hostility' risk making 'two-tier policing' even worse, it has been warned.
The Government is set to scrap its previous plan for an 'Islamophobia' definition in favour of the new phrase in a bid to head off claims that it is creating a blasphemy law by the back door.
But Shadow Home Secretary Chris Philp said the proposed definition would be used by Islamist extremists to secure police action against their critics.
It comes in the wake of a report by the Policy Exchange think-tank, published earlier this week, that said the 'anti-Muslim hostility' term would actually be 'more dangerous' than Labour's previous proposals.
Mr Philp said: 'The Government's plans risk giving the green light to Islamists who would use this definition to ruthlessly shut down criticism of their extremist views.
'The Government's work is a direct threat to free speech and is likely to lead to even more two-tier policing.
'This would mean even more innocent people being harassed for expressing legitimate views or for debating political Islam or even the religion itself.'
He added: 'Free speech is one of the foundations of a democratic society and the Government's plans undermine that.
'I wholeheartedly support the conclusions of this report which outlines those grave threats.'
Director of the Free Speech Union (FSU), Lord Young, said: 'A definition of "anti-Muslim hostility", if it is given the stamp of official government approval and rolled out across public bodies, will have a deeply chilling effect on free speech.
'Anyone criticising something done by Muslims or by an Islamic regime such as Iran would be vulnerable to the accusation that they are motivated by "anti-Muslim hostility".
'The FSU will almost certainly mount a legal challenge should the Government go ahead with this proposal.'
Free speech campaigners have previously argued that special protections for the Muslim faith would prevent legitimate criticism of the religion and risk creating a de facto blasphemy law.
The Daily Mail reported on Thursday how the Policy Exchange report warned: 'A definition of "anti-Muslim hostility" is, in fact, at least as broad and dangerous as one of "Islamophobia," possibly more so.
'And because the term lacks (for now) the same negative political connotations, it is also more deployable by activist groups.
'The dictionary definition of "hostility" includes "not liking" something, "not agreeing" with it or being "opposed" to it.'
The new definition could be exploited by some Muslim groups seeking to influence political debate on issues such as immigration and counter-terrorism, the paper suggested.
Its conclusions were backed by Baroness Falkner, who served as chairwoman of the Equality and Human Rights Commission until last year.
She said the 'dangerous and divisive' proposed definition was 'even worse than anticipated,' and warned it could 'become a free speech and thought control problem'.
Separately, the move could affect plans by police chiefs to scrap 'non-crime hate incidents', which have led to a series of cases which saw members of the public arrested for making what others regarded as offensive remarks.
The report said: 'It is easy to see how a definition could become "non-crime hate incidents Mark Two", at least for non-crimes involving Muslims, used to pressure police forces, local authorities, employers or other bodies to record or sanction incidents of "anti-Muslim hostility".'
Co-author of the Policy Exchange report Andrew Gilligan said: 'This proposed new official definition would be non-crime hate incidents on stilts and would not reduce the grave risks associated with an 'Islamophobia' definition.
'As we set out in our report, the replacement phrase is far broader in scope and would concentrate rather than dilute the damage that could be caused to free society.
'This dangerous definition would risk being used as a tool by Islamist and political Islam groups to intensify "two-tier policing".
'It would become a whole new mechanism by which they could terminate debate and silence their critics.'
Communities Secretary Steve Reed is finalising the new definition based on recommendations from an advisory group, headed by Dominic Grieve, the former Tory attorney general.
Plans for a shift away from 'Islamophobia' to 'anti-Muslim hostility' were partially leaked to the BBC in December but are yet to be formally published.
It is unclear whether the proposal has won consensus within Labour.
Last September Shabana Mahmood, Britain's first female Muslim Home Secretary, indicated her reservations about the move.
She said: 'It might be the case that everyone is safer when you're all subject to the same law.'
