Image caption The Most Reverend Justin Welby said struggling families would be hard hit

The Archbishop of Canterbury Justin Welby has warned changes to the benefit system could drive children and families into poverty.

He said society had a duty to support the "vulnerable and in need".

His comments backed an open letter from bishops criticising plans to limit rises in working-age benefits and some tax credits to 1% for three years.

The Department for Work and Pensions said changing the system will help get people "into work and out of poverty".

Shadow home secretary Yvette Cooper told the BBC's Andrew Marr Show that Archbishop Welby was "absolutely right" to speak out and described the proposals as "immoral".

Civilised society

The welfare bill will be debated in the Lords next week and bishops in the house have tabled an amendment in an attempt to see child-related benefits made exempt.

The letter in the Sunday Telegraph from the 43 Church of England bishops, which calls on politicians to "protect" children and families, has also been supported by the Archbishop of York, the Most Reverend John Sentamu.

When times are hard, that duty should be felt more than ever, not disappear or diminish Justin Welby, Archbishop of Canterbury

In a statement, Archbishop Welby said: "Politicians have a clear choice. By protecting children from the effects of this bill, they can help fulfil their commitment to end child poverty."

He said planned benefit changes, which would cap rises in welfare payments for the next three years, would exact a large price on families.

The archbishop said a "civilised society" had a duty to support the vulnerable.

"When times are hard, that duty should be felt more than ever, not disappear or diminish," he said.

He said the current system recognised rising costs of food, fuel and housing by giving benefit rises in line with inflation.

"These changes will mean it is children and families who will pay the price for high inflation, rather than the government," he said.

The statement is his first major intervention in political life since he was named in his new role in November. He is due to be formally enthroned at Canterbury Cathedral on 21 March.

Analysis It is a signal that the new Archbishop of Canterbury will not shy away from challenging political decisions. Archbishop Welby has made what looks like a highly charged intervention in a politically sensitive debate very early in his tenure. It is not the first time that the government has been criticised by an archbishop or by the clergy. But the Church of England has helped the government's critics to make their case against the benefit cap. It comes just over a week before the chancellor delivers his budget. Some elements of it may depend on what ministers see as these crucial welfare savings. The bishops themselves are hoping to press for a vote on an amendment which would exempt child-related benefits from the government's proposals. That could result in an embarrassing defeat for ministers. But Whitehall sources say the Anglican church challenge will be hard fought. Ministers argue that they are protecting some of those lowest paid but insist they have to get the welfare bill down.

'Fairness test'

The Rt Rev David Walker, Bishop of Dudley and one of the signatories of the letter, told the BBC that bishops had resorted to writing to the press "because we had tried everything else".

He said: "These changes are the not the right ones of our country. It is a bad test of a country's fairness that it rewards the wealthy and it makes the poorest take the heat of the burden when we've got a recession."

The BBC's religious affairs correspondent Robert Pigott said Archbishop Welby's comments would be interpreted as a rebuke to ministers.

It suggests tackling poverty will be a priority for him as the Church's leader, our correspondent added.

BBC political correspondent Tim Reid said if the bishops' amendment to the bill was successful and later approved by MPs, it would cause difficulty for the chancellor as he would then need to rethink his figures for the overall welfare budget.

In their letter, the bishops said they were concerned 200,000 children could be pushed into poverty.

"Children and families are already being hit hard by cuts to support, including those to tax credits, maternity benefits and help with housing costs," they write.

"They cannot afford this further hardship penalty. We are calling on the House of Lords to take action to protect children from the impact of this bill."

The letter is in support of a campaign by the Children's Society which it said has also received support from the Roman Catholic and Methodist Churches, the Baptist Union, the United Reform Church and the Evangelical Alliance.

'Mums not millionaires'

Speaking to the BBC, Ms Cooper said Labour was against the 1% cap.

Media playback is unsupported on your device Media caption Shadow home secretary Yvette Cooper described the government proposals as "immoral"

"They should just have the benefits go forward linked to inflation this year," the shadow home secretary said.

"You could do it by paying for it by restricting pensions tax relief on the very highest earners. That would be a fair way to help everybody."

She added new mothers would lose £180 a year in maternity benefits as a result of the 1% cap.

"I think it is pretty simple on Mother's Day to say that the government should help mums not millionaires," she said.

Liberal Democrats' president Tim Farron said the party had been working to ensure the poor would be protected under the coalition's plans and the archbishop's intervention was "an immensely helpful one in strengthening [its] hand to fight for a fairer deal".

Ex-Lib Dem leader Paddy Ashdown defended the coalition's record on helping poorer families but said: "I don't think we can get ourselves out of the economic mess that we're in without people having a price to pay".

A Department for Work and Pensions spokesperson said "simply increasing benefits" would not tackle poverty.

"For too long the welfare system has kept families trapped in a cycle of benefit dependency and made it impossible for many to contemplate moving into work and off benefits.

"We are fundamentally changing the system so people are helped into work and out of poverty, whilst providing support for those where work is not a realistic option.

"Benefits have risen twice as fast as wages over the past five years, and even in these difficult economic times, they will continue to rise each year."