The Christian Institute and the National Secular Society are speaking out on behalf of an 89-year-old atheist who claims he was threatened with arrest for posting in the window of his home a sign suggesting religion is a fairy tale. The organisations see the case as an example of why the Public Order Act needs to be revised.

John Richards is claiming that when police became aware of the sign hanging in a window of his home in Lincolnshire, they told him he could be arrested if it drew any complaints. The sign Richards placed for passersby to see reads: "Religions are fairy stories for adults."

"I am an atheist and I feel people are being misled by religion. I wanted to show people that if they thought they were alone there was at least one other person who thought that," Richards told local publication the Boston Standard last week.

"The police said I could be arrested if somebody complained and said they were insulted, but the sign was up two years ago and nobody responded or smashed the window," the retiree said.

Richards was reportedly told that the sign could be in violation of Section 5 of the Public Order Act, which prohibits causing "harassment, alarm or distress" to other people.

The part particular to Richards says that a person is guilty of an offense and could be arrested without a warrant if he "displays any writing, sign or other visible representation which is threatening, abusive or insulting, within the hearing or sight of a person" who would find the display distressing.

Although he admits that the authorities did not explicitly threaten to lock him up for the sign, the elderly atheist said he felt threatened nonetheless.

"I accept that the police emphasised the words could lead to an arrest but the implication is a threat to free speech which surely should be fought," Richards said.

Commenting on his case, The Christian Institute agreed with Richards that the suggestion of an arrest was damaging enough.

"It is an intimidating experience for law-abiding people to be told by officers that their beliefs are against the law. Even where no arrest is ever made, it exerts a dangerous chilling effect on free speech," said Simon Calvert of The Christian Institute.

Lincolnshire Police issued a statement insisting Richards was never threatened with arrest. The statement noted that an arrest would only be made if a person found in violation of the law refused to remove a threatening, abusive or insulting sign.

"If a complaint is received by the police in relation to a sign displayed in a person's window, an officer would attend and make a reasoned judgement about whether an offense had been committed under the Act," read the statement republished by the Boston Standard.

The Christian Institute, insisting that the authorities often misuse the Public Order Act, suggested that many Christians in Britain would "sympathise strongly" with Richards.

The organisation noted that Richards case was another example of why some citizens, including lawmakers, thought it was high time to revamp the Act, especially for the term "insulting" to be cut from the legislation.

The organisation, working with the National Secular Society, has launched the "Reform Section 5" campaign to push for an amendment to the code.

On the campaign website, it reads: "At best, Section 5 is the legal embodiment of a well-meaning bureaucrat, and at worse it is used as a way of silencing those who the authorities don't agree with – be they religious preachers, political activists or protesters."

Richards revealed in a report from The Sun that the police only became aware of the sign he had posted when he sent them an email about the display and his concerns about being found in violation of the code.

While expressing his wonder at how one could actually prove his or her "distress" in such cases, the atheist suggested that exercising free speech has its risks but applies to all.

"In an email to the police I did say that I could be distressed by signs outside a church saying 'Jesus Saves'. But I don't go around saying that they should be taking them down. Freedom of speech means anyone can express their views," he said.