It is widely seen as a make-or-break moment for the Anglican church, with human sexuality among many important issues on the agenda.
The leaders of the 38 Anglican provinces will be at the gathering in Canterbury on 11-16 January, for the gathering which was called by the Archbishop of Canterbury in September last year.
Speaking ahead of the summit, Justin Welby said: "As we approach the Primates' meeting we need to recognise that we're going to be dealing with some very, very difficult issues - within the life of the Anglican Communion but also hugely difficult issues that are affecting the whole church of Christ and our whole world.
"What I would ask people to pray for more than anything else is wisdom and love."
There were reports that Church leaders from Africa and Asia were threatening to walk out of the talks, unless American bishops drop their support for same-sex marriage.
The agenda for the Primates' Meeting will be set by common agreement, but it is likely to also include the issues of religiously-motivated violence, the protection of children and vulnerable adults and the environment.
Most Revd Justin Welby gave further details on why he would like Christians to pray for love and wisdom.
"That the love of Christ for each of us, for each of us who are sinners, each of us who fail, will so overwhelm us that we are able to love each other as we should.
"And wisdom that we may know the call and purpose of God and in love and wisdom serve his world in the way he calls us to."