The Commissioner of the Metropolitan Police is being asked to take steps to ensure officers do not arrest street preachers who call homosexuality a sin.
The appeal from Christian Legal Centre (CLC) follows the arrest this month of Tony Miano in Wimbledon after police received a complaint of homophobia from a member of the public who reportedly told him to "f*** off".
Miano, from the US, had been preaching on sexual immorality from 1 Thessalonians 4: 1 - 2 at the time.
He was arrested under Section 5 of the Public Order Act 1986 and taken to the police station where he was interviewed and fingerprinted. After seven hours, he was released without any further action being taken.
The CLC said some officers were wrongly using their powers to intimidate or silence Christians who have spoken of homosexuality as a sin, and that arrests on similar grounds may become more common following the legalisation of gay marriage.
In its letter to the police chief, CLC said guidance needed to be issued to officers clarifying that it is not unlawful to state that homosexuality is a sin unless it is in a manner that stirs up religious hatred.
Andrea Minichiello Williams, CLC chief executive said: "We would like to see clear guidance from the Metropolitan Commissioner that Christians preaching from the bible that homosexual conduct is sinful is lawful free speech.
"The police officers in this case found that the complainant saying 'f*** off' was non-offensive and lawful but that the evangelist's preaching from the Bible was an arrestable offence.
"Free speech is under threat and we need to protect it. We hope that the Commissioner will take a lead in this."
In the letter, the CLC also calls for the officers who arrested Miano to be disciplined.
"Evangelist Tony Miano did no more than suggest that homosexual practice is a sin (not sexual orientation). As you might be aware, this is the position adopted by all the major churches in the United Kingdom," the letter reads.
"Furthermore, the Police officers appear to regard the term 'f*** off' as non-offensive and lawful (as expressed by the Complainant); and the preaching from the Bible as offensive."
The CLC also criticised police for questioning Mr Miano about his personal beliefs during the interview at the station, which it argued was unlawful on the grounds of being intrusive to 'private life' under Article 8 of the European Convention.
"From the transcript it appears as though the officers are convinced that if someone perceives there to have been a 'homophobic incident' then they believe an offence has occurred and therefore the speaker can be arrested," the CLC said.
"We remain of the view that no reasonable Police officer would arrest a Christian for preaching a Bible message in the circumstances of this case, nor pursue such an offensive line of questioning."