In an article written for the Mail on Sunday newspaper, Welby, spiritual leader of the Anglican Communion of millions of Christians globally, said Britons remain deeply divided over Brexit after negotiations got under way last week.
May is struggling to repair her authority after losing the Conservatives’ parliamentary majority in the June 8 election.
She faces pressure from inside and outside her party to temper her plans for a clean break with the EU, with some politicians pushing for Britain to stay in the bloc’s customs union or even its broader single market.
“We must develop a forum or commission or some political tool which can hold the ring for the differences to be fought out, so that a commonly agreed negotiating aim is achieved,” Welby wrote. “It could not bind parliament, but well-structured it could draw much of the poison from the debate.”
He said Britain needed unity at a time when it was dealing with the after-effects of a devastating fire in a London apartment block and Islamist militant attacks in the capital and Manchester.
“A country united after Brexit is essential if we want a country that is resilient under the threats we face, capable of ensuring that the victims of Grenfell Tower are cared for and its lessons learned, and courageous in making our way in the post-Brexit world,” he wrote.
“The decisions we make over the next two years will have an impact for generations to come.”