Rowan Williams has joined Cardinal Keith O'Brien, Archbishop of St Andrews and Edinburgh, among other church leaders, in signing a declaration which says reducing dependence on fossil fuels is "essential" to Christian discipleship.

The launch declaration comes on Ash Wednesday, the start of Lent, traditionally a period of penitence, prayer and reflection for Christians in the run-up to Easter. The declaration will be marked by a service in central London this evening.

Many Christians receive a mark of ashes on the forehead as a token of penitence and mortality.

"Continuing to pollute the atmosphere when we know the dangers goes against what we know of God's ways and God's will," the declaration said.

"We are failing to love not only the earth, but our neighbours and ourselves, who are made in God's image. God grieves over the destruction of creation and so should we."