The backlash against InterVarsity's policy to terminate workers who support gay marriage cranked up on Tuesday as influential authors and alumni of the Christian organisation slammed the decision.

In a public letter, at least 40 authors from the ministry's publishing house wrote to chief executive Tom Lin and urged him to allow for different views. It comes after InterVarsity Christian Fellowship (IVCF) said it expected staff who disagree with its stance on a range of issues including gay marriage to admit their conflict and leave.

IVCF is one of the largest evangelical Christian organisations on US college campuses, with 667 branches.

An 18-month study into LGBT ministry concluded with the policy of "involuntary terminations" for anyone who admits they disagree with the company's position.

Staff are expected to come forward if they disagree with a 20-page theological summary that includes opposition to same-sex relationships, sex before marriage, divorce and masturbation.

"We're trusting their integrity that they'll resign rather than continue to work with an organization that disagrees with them," said vice-president Greg Jao. "We framed it as an involuntary termination, even though staff are self-disclosing. We are trying to acknowledge that they would not have chosen this except for the fact we have clarified and reiterated our position."

But the heat has stepped up as the letter signed by several household names such as Shane Claiborne, David Dark, Christena Cleveland, and Chris Heuertz expressed "deep sorrow" at the decision.

The signatories say they "do not all share the same theological or political views" but "are united in our concern for the dignity and care of our fellow Christians whose jobs are threatened by your policy," according to Religion News Service (RNS).

The full copy of the letter was sent to RNS. Since its original publication more signatures have been added with the rolling list frequently updated.

It urged IVCF to "revoke the 'involuntary termination' policy and replace it with one that allows your staff to conscientiously object. Allowing IVCF staff to remain in community despite not completely agreeing with a particular view of sexuality and marriage will demonstrate the inclusive and open-armed spirit of Christ."

The letter continued: "The Christian community is in desperate need of healing at this moment, and we believe you can help lead this effort."