In his first major political move since taking office, the spiritual leader of the world's Anglicans attacked Britain's government over welfare on Sunday, saying further cuts to welfare benefits would drive thousands of children into poverty.
By wading into one of the most emotive debates in Britain, Justin Welby underscored his resolve to take an active stance on social issues where he believes the Church should have more say.
In a statement on Sunday, Welby said the government's planned changes to the benefit system would hit many families hard at a time when Britain already risks tipping into its third recession in four years.
"Politicians have a clear choice. By protecting children from the effects of this bill, they can help fulfil their commitment to end child poverty," Welby said in a statement.
"These changes will mean it is children and families who will pay the price for high inflation, rather than the government," he said, adding the move could push 200,000 children into poverty.
Welby was named in November after his predecessor, Rowan Williams, resigned after a decade in the job. He is due to be enthroned formally at Canterbury Cathedral on March 21.
MPs are due to debate changes to the welfare bill later in the month, including proposed changes that could cap benefit rises at one percent a year until 2016.
Government officials say the changes are needed as part of efforts to cut the deficit, ease the burden on tax payers and make welfare costs sustainable in the longer term.
The opposition, which has long attacked the ruling coalition over its austerity programme, seized on Welby's remarks to make their own point heard. Opposition politician and shadow home secretary Yvette Cooper said Welby was right.
"They should just have the benefits go forward linked to inflation this year," she told the BBC. "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."
In an open letter published separately in Britain's Sunday Telegraph newspaper, 43 Anglican bishops said the cuts would have a "deeply disproportionate" effect on children.
"A third of all households will be affected by the Bill, but nearly nine out of 10 families with children will be hit," it said. "Children and families are already being hit hard by cuts to support... They cannot afford this further hardship penalty."
(Editing by Giles Elgood)