An open letter to Home Secretary Sajid Javid is urging him to reject a proposed definition of Islamophobia over fears that it will shut down legitimate debate around Islam and be used to shield extremists.

A report by the all-party parliamentary group (APPG) for Muslims says, "Islamophobia is rooted in racism and is a type of racism that targets expressions of Muslimness or perceived Muslimness."

The definition has been formally adopted by the Labour Party, the Liberal Democrats, the Scottish Conservatives Plaid Cymru and the Mayor of London. 

The open letter to Mr Javid acknowledges an "urgent need" to address hatred and acts of violence against Muslims, such as the recent attack in Christchurch, but warns against any "uncritical and hasty" adoption of the APPG's definition of Islamophobia. 

It argues that the definition is "vague and expansive" and has been adopted without proper consideration of the possible negative consequences for freedom of expression, and academic and journalistic freedom. 

It further warns that the definition will "undermine social cohesion - fuelling the very bigotry against Muslims which it is designed to prevent", and could even lead to harmful practices or extremism going unreported.

"We are concerned that allegations of Islamophobia will be, indeed already are being, used to effectively shield Islamic beliefs and even extremists from criticism, and that formalising this definition will result in it being employed effectively as something of a backdoor blasphemy law," the letter reads.

It has been signed by a wide range of academics, faith leaders, journalists, politicians and campaigners, including Professor Richard Dawkins, Lord Alton, Lord Singh, Baroness Cox, Bishop Michael Nazir-Ali, gay rights campaigner Peter Tatchell, the President of Hindu Forum of Britain Trupti Patel, Christian Concern's Head of Policy Tim Dieppe, and National Secular Society CEO Stephen Evans.  

The letter continues: "Evidently abuse, harmful practices, or the activities of groups and individuals which promote ideas contrary to British values are far more likely to go unreported as a result of fear of being called Islamophobic.

"This will only increase if the APPG definition is formally adopted in law. We are concerned that the definition will be used to shut down legitimate criticism and investigation." 

The signatories argue that existing law already provides sufficient protection against attacks and unlawful discrimination on the basis of religion.  They propose that the term 'anti-Muslim hatred' be adopted instead.  

"No religion should be given special protection against criticism," they say. "Like anti-Sikh, anti-Christian, or anti-Hindu hatred, we believe the term anti-Muslim hatred is more appropriate and less likely to infringe on free speech," they say.

"A proliferation of 'phobias' is not desirable, as already stated by Sikh and Christian organisations who recognise the importance of free discussion about their beliefs."

They added: "Rather than helping, this definition is likely to create a climate of self-censorship whereby people are fearful of criticising Islam and Islamic beliefs.

"It will therefore effectively shut down open discussions about matters of public interest. It will only aggravate community tensions further and is therefore no long term solution." 

The Government has said the definition needs to be given more consideration. 

A spokesperson said: "Any hatred directed against British Muslims and others because of their faith or heritage is utterly unacceptable."

The spokesperson added: "This is a matter that needs further careful consideration."

Supporters of the APPG's definition include former Conservative party chairman Sayeeda Warsi, who has dismissed concerns raised by Martin Hewitt, chairman of the National Police Chiefs Council, as "irresponsible scaremongering". 

The Muslim Council of Britain told Buzzfeed that it was "truly astonishing the Government thinks it knows better than Muslim communities".

"If this free speech rationale is true, it would mean that the government believes that defining the racism that targets Muslims or expressions of Muslimness somehow impinges on free speech. Defining antisemitism does not do so, but defining Islamophobia does," the organisation said.