A law banning "insulting" language and behaviour is strangling free speech and should be scrapped,a senior Tory MP has demanded, after a student was held for calling a police officer's horse "gay".

Former shadow home secretary David Davis said Section 5 of the 1986 Public Order Act was having a "terrible, chilling effect on democracy".

Polling suggests almost two-thirds of MPs back Davis, according to a campaign which has brought together religious and secular groups along with human rights and minority organisations.

Under the legislation, the use of "insulting words or behaviour" is outlawed, but opponents say there is too little clarity of what that includes, leading to spurious arrests.

One teenage boy was arrested for holding a "Scientology is a dangerous cult" placard and a student was held for telling a police officer his horse was "gay", they said.

While it was right to protect people against unjust discrimination and the incitement of violence against them, the campaign said, insulting behaviour was open to too much interpretation.

The campaign is using the slogan "Feel free to insult me".

Davis said repeal was "vital to protecting freedom of expression in Britain today".

"Who should decide who is insulted? The police? A judge? The truth is that Section 5 is having a terrible, chilling effect on democracy today."

A ComRes poll commissioned by the campaign suggested he would have the backing of a clear majority of MPs - with 62% saying it should not be for the state to ban insults.

Only 17% thought repeal would undermine the ability of the police to protect the public and one in five believed it would penalise minorities.

The campaign has secured a rare alliance of secularist and Christian groups.

Keith Porteous Wood, of the National Secular Society, said: "Free speech is not free if it is available only to some and not others.

"Secularists, in defending free expression, must ensure that the law is fair to everybody and argue equally for the right of religious and non-religious people to freely criticise and exchange opinions without fear of the law - unless they are inciting violence."

The Christian Institute's Simon Calvert said: "Churches around the world find themselves in constant friction with aspects of the cultures in which they live. So free speech is vital to us all.

"Britain's historic civil liberties were often hammered out amidst controversy over freedom to preach without state interference. Christians know first hand why free speech is precious and this is why the Christian Institute is pleased to join people across the political and philosophical spectrum to help bring about this simple but important change."

Other groups signed up to the campaign include Big Brother Watch, the Freedom Association and gay rights campaigner Peter Tatchell, who has himself been arrested under Section 5.
Tatchell said: "The open exchange of ideas - including unpalatable, even offensive, ideas - is a hallmark of a free and democratic society."

ComRes surveyed 154 MPs between February 14 and March 14 by self-completion postal or online questionnaire. Data were weighted to reflect the composition of the House of Commons.