The Evangelical Alliance, which represents 3,500 evangelical churches in the UK, has written to Prime Minister Boris Johnson urging him not to ban so-called conversion therapy.
In their letter, they argue that a wide-ranging ban would restrict religious freedoms and criminalise church teaching.
It states: "An expansive definition of conversion therapy, and a ban along such lines, would place church leaders at risk of prosecution when they preach on biblical texts relating to marriage and sexuality.
"It would place ministry leaders at risk of arrest for encouraging young people to maintain chastity until marriage.
"And it would criminalise a member of a church who prays with another member when they ask for prayer to resist temptation as they are attracted to someone of the same sex but do not wish to act on it."
The Evangelical Alliance has always defended the practice of trying to change or suppress a person's sexuality, with a declaration on its website stating: "We welcome and support the work of those individuals and organisations who responsibly seek to help Christians who experience same-sex attraction as in conflict with their commitment to live in accordance with biblical teaching.
"Some may seek and experience changes in the strength or direction of their same-sex attractions."
The Evangelical Alliance says it has decided to go public as debate intensifies over whether the practice should be banned in law.
In an interview, the Evangelical Alliance's UK director Peter Lynas told ITV News the government must define conversion therapy before banning it, so the group can engage in discussions about how to protect religious freedom.
"We obviously don't want to see people harming themselves, but we also have to be able to teach what the Bible says, that's what freedom of religion entails," he said.
Last week, three of the government's equalities advisers resigned from their roles, accusing ministers of creating a "hostile environment" for LGBT+ people, with fears a promised ban on conversion therapy was being dropped.
In the aftermath, equalities minister Liz Truss recommitted the government to banning the practice, as originally promised in 2018.
A debate now rages over how to define both conversion therapy and the scope of any ban.
Conservative MPs have been called to a meeting with the equalities minister on Tuesday afternoon to discuss so-called conversion therapy.
In its letter, the Evangelical Alliance says it condemns the most extreme forms, including corrective rape and electroshock therapy. But it argues that these are already illegal.
Campaigners want a ban to go further to cover more common forms of conversion therapy involving spiritual therapies and religious prayer.
ITV News has previously revealed how some such practices have left victims feeling suicidal.
But the Evangelical Alliance argues in its letter that legislating in this area would impinge on religious freedoms.
"For evangelical Christians, the teaching of the Bible is clear that sexual activity is restricted to monogamous marriage between one man and one woman.
"For Christians who hold to this biblical teaching, it is essential that those who experience same sex attraction are free to pursue and receive support to help them live in accordance with their beliefs."
Paul Bayes, the Bishop of Liverpool, hit out at the group over its letter to the prime minister.
He said the Evangelical Alliance is "mistaken" in its letter, adding: "LGBTI+ orientation/identity is not a sickness, not a crime and not a sin."
In his tweet, he said conversion therapy "damages people", and he hopes the government will "stay faithful to its commitment to a ban".
Steve Chalk, the founder of Oasis Church, which is a member of the Evangelical Alliance, also took a stance against the letter.
In a statement, he said: "Oppressive practices are most dangerous when they are dressed up as ‘pastoral support’... based on the misguided and damaging belief that the sexual or gender orientation of LGBT+ people is an ‘illness’."
Responding to the letter, Jayne Ozanne, who was among the advisers to resign last week, told ITV News: "Here is clear proof that those who wish to continue the practice of conversion therapy wish to do so with impunity, deaf to the cries of the multitude of people they have harmed.
Listen to Paul Brand's Acting Prime Minister podcast:
"The government has a clear choice - to side with the perpetrators of conversion therapy or to protect vulnerable victims. Freedom of religious belief is only a freedom up until the point that it does no harm."
ITV News understands that several Conservative MPs take a similar view to the Evangelical Alliance and are privately lobbying the government not to ban conversion therapy.
But there are many in Parliament who are vocally in favour of outlawing the practice.
Yesterday, eight political parties wrote to the equalities minister urging her to deliver on the government's promises, with their letter organised by LGBT+ Conservatives.
Labour MP Stephen Doughty labelled the Evangelical Alliance's letter to the PM "deeply saddening and concerning".
"Christ implores us to love God, and love one another. Please can we just do that?" he asked.
Fellow Labour MP Ben Bradshaw said the Evangelical Alliance "brings shame on my religion".
The government has promised to bring forward measures soon, with an announcement expected within weeks.