Sixty Church of England bishops along with leaders of other religious groups, are urging ministers to rethink the two-child benefits cap.
In a letter to the Times, they say the policy is likely to tip an extra 200,000 children into poverty.
Changes limiting some benefits to the first two children in a family came into effect last year.
The government says parents on benefits should face the same financial choices as those in work.
Under the current rules, parents can claim child tax credits or universal credit only for their first two children unless there are special circumstances.
Campaigners say the policy could leave some families almost £3,000 a year worse off.
Among the signatories to the letter are senior representatives from Jewish and Muslim groups as well as other Christian organisations.
- The new Bishop of London, the Right Reverend Sarah Mullally
- Harun Khan, secretary general of the Muslim Council of Britain
- Gillian Merron, chief executive of the Board of Deputies of British Jews.
They write: "It is a grave concern that there are likely to be mothers who will face an invidious choice between poverty and terminating an unplanned pregnancy."
They warn that the policy "conveys the regrettable message that some children matter less than others, depending on their place in the sibling birth order".
They suggest that many families will have taken decisions on family size while they were still earning enough to support their children but may now have to claim benefits because of redundancy, bereavement or illness.
'Simply not right'
The Right Reverend Paul Butler, Bishop of Durham, said: "A combination of low pay, unstable jobs and high housing and living costs are locking families in a daily struggle to put food on the table.
"It is simply not right that some children get support and others don't.
"We share a moral responsibility to make sure that everyone in our country has a decent standard of living and the same chances in life, no matter who they are or where they come from.
"The government has an opportunity to right this wrong by removing its two-child limit policy. We urge the prime minister to address this burning injustice."
A report published with the letter by the End Child Poverty Coalition, Child Poverty Action Group and the Church of England says the policy's full impact has yet to be seen.
Some 160,000 families with newborn babies are already up to £2,780 a year worse off than if their youngest child had been born in the previous year, says the report.
And, from February 2019, the two-child limit will also apply to families with three or more children who make a new claim for universal credit, irrespective of when their children were born.
By 2020, an estimated 640,000 families, including around 2 million children, will be affected, it adds.
Child Poverty Action Group chief executive Alison Garnham said the link between need and benefit entitlement had been broken by the policy.
"A year in, the government should reconsider this policy before more families are pulled below the poverty line."
A Department for Work and Pensions spokesman said: "We have always been clear that this policy will be delivered in the most effective, compassionate way, with the right exceptions and safeguards in place but it's right that people on benefits have to make the same financial choices as those supporting themselves solely through work."