Christian groups have joined forces with civil liberties campaigners and secular organisations to remove the word "insulting" from the 1986 Public Order Act.

Currently the law outlaws the use of "insulting words, or behaviour", but groups – including the Evangelical Alliance – worry that the word restricts free speech and penalises campaigners, protestors and preachers.

The law has been used against a student who told a police officer his horse was gay, a teenager who carried a sign which read "scientology is a dangerous cult" and human rights campaigner Peter Tatchell (pictured) who held a placard denouncing Hizb ut-Tahrir.

Mr Tatchell is a vocal supporter of the Campaign to Reform Section 5 (RS5), which was launched in May by MPs from the three political parties, the Christian Institute and the National Secular Society.

This evening Mr Tatchell will take part in a protest outside the Home Office, calling on home secretary Theresa May to remove the word.

The protest comes ahead of a vote in the House of Lords on Wednesday on the issue.

Mr Tatchell will be joined in his protest by Simon Calvert of the Christian Institute, who is also the RS5 campaign director.

Today the director of public prosecutions Keir Starmer QC said the word "insulting" could safely be removed "without the risk of undermining the ability for the Crown Prosecution Service to bring prosecutions".

Responding, Mr Calvert said: "This is great news that pulls the rug from under the government's chief excuse for resisting reform. The Home Office has continually used the line that if it removes the word 'insulting' then it would not be able to prosecute hooligans and yobs who swear at the police… That is simply not the case.

"We hope that the home secretary Theresa May will listen to country's top prosecutor and agree to reform this over board and unwanted legislation, which has a chilling effect on free speech."

He added: "I don't think any campaign has united such a diverse range of campaigners, MPs from left and right, faith-based groups and secularists. The support for reform is overwhelming.

"It cannot be right for the collective voices of so many to be ignored and we challenge the government to back Lord Dear's amendment in the House of Lords this Wednesday."

Mr Tatchell added: "The legislation is now being used to criminalise huge numbers of people for trivial comments. In 2009 the police used this law 18,000 times, including against people who were expressing their views or beliefs in a reasonable manner that was non-abusive and non-threatening.

“The problem with the current law is the word insulting is completely elastic and subjective, which is why people have arrested for saying a police horse was gay, or that scientology was a cult.

“The longer 'insults' are criminalised, the more people will risk losing their right to freedom of expression. We have cross-party support in parliament and support from campaign groups that are often at odds with each other. It's time for change."

Dr Don Horrocks, head of public affairs at the Evangelical Alliance, said: "We have been urging reform of Section 5 for a number of years and fully support this latest attempt to amend a law which is so open to misuse.

"As we observe increasing threats both in the UK and around the world to close down public expression of views deemed unacceptable by certain sections of society, it is vital that we do everything possible to preserve freedom of speech and freedom to disagree.”