Richard Dawkins, a renowned atheist, had been due to speak at an event hosted by Berkeley-based station KPFA

A row over free speech has broken out between scientist Richard Dawkins and a California radio station after a planned speech was cancelled.

The British biologist, a renowned atheist, had been due to speak at an event hosted by Berkeley-based station KPFA about his latest book.

But the speech was pulled, with organizers citing 'hurtful' statements made by Dawkins about Islam.

It is not known what specific remarks prompted the cancellation, but online commentators have drawn attention to a 2013 tweet in which the writer described Islam as 'the greatest force for evil in the world today'.

Dawkins himself has hit back, saying his issue is with Islamism, and pointing out that he has never been barred from events because of his criticism of Christianity.

A statement from KPFA said: 'We had booked this event based entirely on his excellent new book on science, when we didn’t know he had offended and hurt – in his tweets and other comments on Islam – so many people.

Dawkins (left), had been due to speak about his latest book 'Brief Candle In The Dark' (right)

'KPFA does not endorse hurtful speech. While KPFA emphatically supports serious free speech, we do not support abusive speech.

'We apologise for not having had broader knowledge of Dawkins’s views much earlier. We also apologise to all those inconvenienced by this cancellation.'

In an open letter, Dawkins responded to the radio station: 'If you had consulted me, or if you had done even rudimentary fact-checking, you would have concluded that I have never used abusive speech against Islam.

'I have called IslamISM “vile” but surely you, of all people, understand that Islamism is not the same as Islam.

In 2013, Dawkins branded Islam 'the greatest force for evil in the world today' in a Twitter post

'I have criticised the ridiculous pseudoscientific claims made by Islamic apologists (“the sun sets in a marsh” etc), and the opposition of Islamic “ scholars” to evolution and other scientific truths.'

His letter continued: 'I have criticised the appalling misogyny and homophobia of Islam, I have criticised the murdering of apostates for no crime other than their disbelief.

'Far from attacking Muslims, I understand – as perhaps you do not – that Muslims themselves are the prime victims of the oppressive cruelties of Islamism, especially Muslim women.'

And the 76-year-old fumed: 'I am known as a frequent critic of Christianity and have never been de-platformed for that.

'Why do you give Islam a free pass? Why is it fine to criticise Christianity but not Islam?'