The Bishop of Durham was unveiled as the new Archbishop of Canterbury today and said being nominated for the post was "astonishing and exciting".

The Rt Rev Justin Welby, an Eton-educated former oil industry executive, will become the 105th Archbishop and spiritual leader of the 77 million-strong Anglican Communion.

He told a news conference at Lambeth Palace: "This is the best-kept secret since the last Cabinet reshuffle.

"To be nominated to this post is both astonishing and exciting."

Bishop Welby was the fourth most senior cleric in the Church of England before the announcement that he would succeed Rowan Williams.

No 10 made the unusual move of announcing the appointment on social networking site Twitter, stating: "Downing Street is pleased to announce the appointment of Justin Welby as the 105th Archbishop of Canterbury."

The bishop began his press conference with a prayer and said: "It's something I never expected, and the last few weeks have been a rather strange experience, to put it mildly."

He added: "One of the biggest challenges is to follow a man who I believe will be recognised as one of the greatest Archbishops of Canterbury, Rowan Williams."

And he joked: "On the basis that you should only follow failures, this is a great mistake."

The appointment marks a meteoric career rise for the clergyman who was enthroned as Bishop of Durham only a year ago.

He worked in the oil industry for 11 years before leaving to train for the Anglican priesthood, and was first ordained as a deacon in 1992.

"I was unable to get away from a sense of God calling," he said in an interview.

Several other senior figures in the Church of England were reported as possible contenders to succeed Dr Williams, who leaves after a decade, at the end of this year, to become master of Magdalene College, Cambridge.

They included the Archbishop of York Dr John Sentamu who was named as an early favourite, the Bishop of Norwich the Rt Rev Graham James, and the Bishop of London, the Rt Rev Richard Chartres.

Dr Williams announced in March that he was to leave as Archbishop, saying: "I would like the successor that God would like.

"I think that it is a job of immense demands and I would hope that my successor has the constitution of an ox and the skin of a rhinoceros really."

The bishop said: "It's exciting, because I believe that we are at one of those rare points, where the tide of events is turning, and the Church nationally including the Church of England has great opportunities to match its very great, but often hidden strengths.

"I feel a massive sense of privilege at being one of those responsible for the leadership of the Church, in a time of spiritual hunger, when our network of parishes and churches and schools and above all people means that we are facing the toughest issues in the toughest places."

The new Archbishop described how his experiences with religion have shaped him and his wife Caroline..

He said: "I am touched by the way so many people have contributed to whom Caroline and I have become.

"I learnt a great deal from the companies for which I worked, above all from bosses and colleagues.

"We have been nurtured and shaped as Christians by the churches in Paris and London.

"I had the privilege of serving as a curate amongst wonderful people in Nuneaton and making many mistakes as a rector in Southam.

"Coventry Cathedral opened my eyes to the church overseas and a passion for reconciliation, and Liverpool teased me mercilessly and quietly talked to me.

"Above all the providence of God has surrounded us in so many ways, through tragedy and through joy."