Christian leaders around the world have expressed their sadness and shock at news that Pope Benedict XVI has resigned.

The Vatican announced this morning that the pope plans on resigning the papal office on 28 February because of his old age.

"After having repeatedly examined my conscience before God," Pope Benedict said, "I have come to the certainty that my strengths, due to an advanced age, are no longer suited to an adequate exercise of the Petrine ministry."

He added: "I have had to recognise my incapacity to adequately fulfil the ministry entrusted to me."

Dave Landrum, director of advocacy at the Evangelical Alliance, said Pope Benedict would be missed.

Referring to the papal visit to the UK in 2010, Dr Landrum said: "The pope has played a valuable role in exposing and critiquing how secular humanism is 'building a dictatorship of relativism that does not recognise anything as definitive and whose ultimate goal consists solely of one's own ego and desires'".

Dr Landrum added: "His contribution to affirming the vital role for religion in public life cannot be underestimated. In his speech in Westminster Hall during his successful 2010 visit to the UK he presented our politicians with the great challenge of our age, asking: 'What will be the ethical basis upon which we make our political decisions?' His towering intellect and humble heart will be greatly missed."

Archbishop of Canterbury Justin Welby said that he had a "heavy heart" regarding the news but understood the pope's decision.

He said: "As I prepare to take up office I speak not only for myself, and my predecessors as Archbishop, but for Anglicans around the world, in giving thanks to God for a priestly life utterly dedicated, in word and deed, in prayer and in costly service, to following Christ. He has laid before us something of the meaning of the Petrine ministry of building up the people of God to full maturity.

"We pray that God will bless him profoundly in retirement with health and peace of mind and heart, and we entrust to the Holy Spirit those who have a responsibility to elect his successor."

Dr John Sentamu, the archbishop of York, praised the pope's fearlessness in proclaiming the gospel and challenging culture.

The Revd Ken Howcroft, who represents World Methodism in Rome, said of the resignation: “It is sad when a Christian leader has to lay aside an office in this way. I was present when the pope presided at a service to mark the end of this year’s Week of Prayer for Christian Unity.

"His address was excellent and clear. But he has started to look more frail recently, and there are great challenges urgently facing all our churches. It will be fascinating to be in Rome as a new pope is found.”