The scale of the Government's cuts to welfare is "hard to comprehend", says the head of Church Action on Poverty.
Niall Cooper says the changes to the benefits system will be felt by the poorest and most vulnerable families in the country.
Six and a half million households will lose out as a result of the uprating of most benefits by one per cent, while the abolition of the current system of Council Tax Benefit will bring further difficulty, with the worst affected being single parents who work part-time and depend on childcare, he warned.
Some will see their benefit levels decrease as a result of the introduction of Universal Credit and around 67,000 families will be affected by the benefit cap of £500 per week.
"April 2013 is D-Day for literally millions of people who rely on welfare benefits or tax credits to help make ends meet," said Mr Cooper.
"Most of us find it hard to comprehend the scale of the Government's welfare reform programme. But make no mistake, for millions of families directly affected, the impacts will be real and lasting."
On Sunday, 43 bishops in the Church of England warned in a letter to The Sunday Telegraph that benefit cuts would have a "deeply disproportionate" effect on children.
"About 60 per cent of the savings from the uprating cap will come from the poorest third of households. Only 3 per cent will come from the wealthiest third," they said.
"If prices rise faster than expected, children and families will no longer have any protection against this. This transfers the risk of high inflation rates from the Treasury to children and families, which is unacceptable."
Their challenge is supported by the Archbishop of Canterbury, Justin Welby.
He said: "As a civilised society, we have a duty to support those among us who are vulnerable and in need. When times are hard, that duty should be felt more than ever, not disappear or diminish.
"It is essential that we have a welfare system that responds to need and recognises the rising costs of food, fuel and housing.
"The current benefits system does that, by ensuring that the support struggling families receive rises with inflation.
"These changes will mean it is children and families who will pay the price for high inflation, rather than the Government."