Archbishop says he has seen mass grave of Christians killed by neighbours who said they feared being 'made to become gay'

African Christians will be killed if the Church of England accepts gay marriage, the archbishop of Canterbury has suggested. Speaking on an LBC phone in, Justin Welby said he had stood by a mass grave in Nigeria of 330 Christians who had been massacred by neighbours who had justified the atrocity by saying: "If we leave a Christian community here we will all be made to become homosexual and so we will kill all the Christians."

"I have stood by gravesides in Africa of a group of Christians who had been attacked because of something that had happened in America. We have to listen to that. We have to be aware of the fact," Welby said. If the Church of England celebrated gay marriages, he added, "the impact of that on Christians far from here, in South Sudan, Pakistan, Nigeria and other places would be absolutely catastrophic. Everything we say here goes round the world."

This reasoning has until now been kept private, although both Welby and his predecessor, Rowan Williams, anguished about it in private.

Welby also condemned homophobia in England. "To treat every human being with equal importance and dignity is a fundamental part of being a Christian," he said. Although he continued to uphold what he called the historic position of the church, of "sex only within marriage and marriage only between a man and a woman", he agreed with the presenter, James O'Brien, that it was "completely unacceptable" for the church to condemn homosexual people more than adulterous heterosexual people.

African churches do not share this opinion, and the Anglican churches in both Uganda and Nigeria have given enthusiastic backing to laws which criminalise even the expression of support for gay marriage. Despite these confusions, Welby denied that the church was woolly in its preaching in a testy exchange with the former Conservative cabinet minister Ann Widdecombe, who left the Church of England over its support of female priests in 1992, but phoned in on Friday to attack it. "I think the opponents of women's ordination are wrong theologically," he said.

Welby refused an opportunity to criticise Iain Duncan Smith on welfare reform, but he was unequivocal in support of the church's work with food banks and against inequality. He cited statistics showing that a third of those coming to food banks were entitled to benefits which had not actually been paid and another third were in employment, but for them "the month is a bit longer than the money".

"Whatever the causes, those are the people we are dealing with. They need to be treated with human dignity and they need to be loved. I do want to live in a country where the economy works in a way that means that food banks are no longer necessary," Welby said.

In remarks which showed the clear influence of Catholic doctrine, he said that food, house prices and energy costs were all moral issues that could not be left entirely to the market. "How much you charge for essentials is always a moral issue," he said.