Dr Williams will say there is now a "genuine opportunity" to eradicate extreme poverty when he speaks at a rally in Lambeth Palace, the London home of the Archbishop of Canterbury.

But he will warn that most of the Millennium Development Goals (MDGs) set in 2000 to halve poverty by 2015, will not, as things stand, be fulfilled by then, and risk never being achieved at all.

The Archbishop will be joined in his plea to governments across the world to honour their promises on aid and development by the Prime Minister Gordon Brown.

The rally will follow a colourful "walk of witness and solidarity" through central London highlighting urgent action needed to achieve the MDGs.

The event will be attended by up to 1,500 faith leaders, diplomats, politicians, heads of charities and other groups.

Most of the 670 Anglican bishops attending the Lambeth Conference will take part in the procession, dressed in their robes, and accompanied by their spouses, many of whom will wear national dress for the event.

Leaders from other Christian churches will also attend including Cardinal Cormac Murphy-O'Connor, the Roman Catholic Archbishop of Westminster, and the leader of the Evangelical Alliance, The Rev Joel Edwards.

The Chief Rabbi, Sir Jonathan Sacks, will also be present as will the former Secretary General of the Muslim Council of Britain, Sir Iqbal Sacranie.

Dr Williams described the event as "a poignant public act of commitment by the Anglican Communion and other faith groups to continue to put pressure on those who have the power and resources to help end extreme poverty across the globe."