Dr Rowan Williams is due to deliver a keynote speech on Monday as leaders of Britain's major faiths criticise the Government's response to the refugee crisis as "too slow, too low and too narrow".
He has spoken out in the past, asking leaders to be more compassionate towards those in need.
Last week, the Government announced it had secured enough local authority places to meet its pledge to resettle 20,000 Syrian refugees over the next four years.
Earlier in 2016 former PM David Cameron said 3,000 unaccompanied children registered in France, Italy and Greece would be allowed to come to Britain.
Christian, Muslim and Jewish clerics, as well as other faith leaders, have signed an open letter urging Mrs May to unblock rules preventing refugee families from being reunited in the UK.
Their letter comes after 300 top former judges and lawyers, 120 of the world's senior economists, and 27 charities and NGOs have also written to the Prime Minister asking her to help.
Bishops and members of the House of Lords as well as senior rabbis, imams and senior figures from other faiths signed the letter.
It says: "Under the present immigration rules, a British doctor of Syrian origin could not bring her parents from a refugee camp in Lebanon - even though they were refugees and she could support and house them.
"A Syrian child who arrived alone in the UK could not bring his parents from a refugee camp in Jordan - even if the child were recognised a refugee and even though his parents were themselves refugees.
"Families in these situations can currently be reunited only by resorting to desperately unsafe irregular journeys, sometimes ending in avoidable tragedies."
