The Archbishop of Canterbury has compared the Church of England to a family at war where people go out into the back yard and shout at each other.

Justin Welby is interviewed for a BBC documentary, The Battle for Christianity, to be broadcast tomorrow night. 

Archbishop Welby said: "We are not a political party we are a family, and if part of the family is struggling, you want to encourage that part of the family.

"It's a family that bickers like crazy. Some argue through sulking quietly in different rooms, and others go out in the backyard and shout at each other. Anglicans tend to be more like that latter if I'm really honest."

The documentary shows a trend where growing churches are becoming more confident, lively and assertive than the traditional British model, often preaching conservative Biblical views on issues such as gay marriage.

Archbishop Welby said: "The Church is neither right nor left. It identifies with the Kingdom of God, which is both right and left in different ways and doesn't really fit on a political spectrum." He said the Church at its best is in "constant engagement" with society. 

Rather than decline, the programme concentrates on growing churches such as the new "missional" church Latimer Minster, the Redeemed Christian Church of God and Hillsong, where 8,000 people attend music-intensive worship services at London's Dominion Theatre across four services each Sunday.

Theology professor Robert Beckford, the presenter, looks at how immigration, radical social action, conservative morality and charismatic worship are all changing traditional Christianity in Britain as wider society becomes increasingly secular.

Aaqil Ahmed, head of religion and ethics at the BBC, wrote in the Independent: "Here in the UK we live in what is referred to as post-Christian Europe, a continent where church membership and attendance is going down. Many people still think of themselves as Christian but in Britain in particular, it's clear that traditional church attendance is down and that a drift into secularisation is for most of us nothing new."

He said migration to Britain from Africa in particular has brought versions of Christianity that are more assertive than the Christianity that most Britons have grown up with. "The assertiveness of these different forms of Christianity are often at odds with more liberal beliefs around issues such as same-sex marriage and blasphemy," he said.

Ahmed wrote: "There are many who would say there is a battle for the soul of Christianity in Britain. It's a fair point but I don't think it's a battle that will necessarily result in a winner. This is more about the arrival of more diversity in Christianity in Britain."

He said this meant Christianity "is not in terminal decline" as many would argue. "Christianity may have been pronounced to be at death's door in the last century, but now it's firmly back in the public space. How we deal with that is the real battle for Christianity here in the UK."

The Battle For Christianity will be broadcast on Tuesday 22 March, 10.45pm to 11.45pm