The Christian Institute is questioning the government's decision to shorten its consultation on a proposed conversion therapy ban.
The consultation on banning so-called conversion therapy ends on 10 December after just six weeks - half the usual 12-week period.
The Christian Institute is calling on the government to let the consultation run for the standard 12 weeks.
The organisation recently launched its Let Us Pray campaign urging Christians to participate in the consultation and pray that Christian parents and ordinary church activities like prayer will not be criminalised.
Simon Calvert, spokesman for the campaign, said: "The more people hear about these proposals, the more concerned they are.
"Feminists are rightly worried about the impact on a parent's ability to challenge the narrative that their gender-confused child is 'trapped in the wrong body'.
"Church groups are concerned an over-broad ban would be used to 'punish' them for having 'the wrong views' about sexuality."
The government's own guidance on consultations warns that "consulting too quickly will not give enough time for consideration and will reduce the quality of responses".
Concerns over the six-week period have been expressed by No 10 adviser Nikki da Costa who told The Times that the government was trying "to get a good news story" in time for next year's government-backed LGBTQ equality conference.
"There's no reason why the government can't take a few more weeks, even a couple months to get this right," said da Costa.
Conservative Peer, Baroness Jenkin of Kennington, tweeted that colleagues in the House of Lords have expressed concern over the six-week consultation period.
The Christian Institute has threatened legal action if the government opts for a broad definition of conversion therapy that includes the ordinary work of churches.
Calvert said the issue was "too important to rush".
"They have to be very careful to get this right. Their proposals appear quite focused in principle, but the consultation paper lacks detail," he said.
"We still don't know how the new law will be worded. So we don't know how the Government will keep its pledge not to criminalise pastors and parents who don't embrace 'Stonewall law' on LGBT issues."
He added, "Allowing the consultation to run the normal 12 weeks would give all sides more opportunity to consider and comment on the proposals and more time for officials to draw up watertight plans."