In a thinly-veiled attack on the Tony Blair era, when Alastair Campbell, the then communications director in Downing Street, said "we don't do God", Mr Alexander warned that people should have the courage to speak up for Christians without fear of causing offence.

Mr Alexander, the shadow foreign secretary, spoke out to voice his concerns about growing harassment and attacks suffered by Christians in the Middle East.

Writing in the Telegraph, he warns that the mounting persecution of Christians is a "story that goes largely untold", describing those who have spoken out on the issue, including the Prince of Wales, as lone voices.

Christianity beginning 'to disappear' in its birthplace, warns Prince of Wales 17 Dec 2013

"Across the world, there will be Christians this week for whom attending a church service this Christmas is not an act of faithful witness, but an act of life-risking bravery. That cannot be right and we need the courage to say so," Mr Alexander says.

"In the UK today, perhaps through a misplaced sense of political correctness, or some sense of embarrassment at 'doing God' in an age when secularism is more common, too many politicians seem to fear discussing any matters related to faith."

He adds: "People of all faiths and none should be horrified by this persecution. We cannot, and we must not, stand by on the other side in silence for fear of offence."

Mr Alexander says persecution of Christians should be treated in the same way as anti-Semitism or Islamophobia.

His intervention comes after the Prince of Wales said he had become "deeply troubled" by the plight of Christians in the Middle East.

Last week, the Prince warned that Christianity was beginning to "disappear" from its own birthplace, as its communities are being "deliberately targeted" by Islamist militants.

Days earlier, a report by The Telegraph highlighted the targeting of Christians by al-Qaeda in Iraq, which has seen the sporadic bombings of churches and killings of priests. One priest warned that thousands of churchgoers had fled one suburb of Baghdad as Christians "feel they are strangers in their own land".

Mr Alexander, who was international development secretary under Gordon Brown, warns that Christians are also under threat farther afield in countries including Nigeria and Pakistan.

The MP says that his own denomination, the Church of Scotland, of which his father was a minister, has felt "very personally" an attack on a church in the Pakistani city of Peshawar.

The mother, nephew, niece, two uncles and other friends and relatives of the Rev Aftab Gohar, one of the Church's ministers, were among 122 people killed in the attack in September.

Since leaving Downing Street, Mr Blair, who converted to Roman Catholicism, has said that he was not open about his faith while in office because he feared voters would think him a "nutter".

However, he said that his faith was "hugely important" in influencing his decisions as prime minister.

Mr Alexander's emphasis on his own faith also contrasts with the self-proclaimed atheism of Ed Miliband, the current Labour leader.

Shortly after his election as leader, Mr Miliband, who is of Jewish descent, was asked in an interview if he believed in God. "I don't believe in God personally, but I have great respect for those people who do," Mr Miliband replied.

David Cameron, the Prime Minister, who describes himself as an "active member" of the Church of England, has said his faith "sort of comes and goes" like the patchy reception of Magic FM in the Chilterns.

Mr Alexander praised Baroness Warsi, the faith minister, for making a speech last month drawing attention to a "mass exodus" of Christians from the Middle East.

He also praised Jim Shannon, a Democratic Unionist Party MP, for leading a parliamentary debate on the issue this month.

Mr Alexander's comments echoed a warning by Lord Sacks, on his retirement as chief rabbi, that the plight of Christians in the Middle East was receiving virtually no attention in public life.

The peer compared the violence faced by Christians in Egypt, Syria and Iraq to the exodus of Jews from Arab countries in 1948, when the establishment of the Jewish state was followed by anti-semitic persecution.

Mr Alexander questioned why "given the scale of the suffering" such figures were still "such lone voices".

He called on the Government to use its place on the UN Human Rights Council, which it will take up in March, to "speak up for religious freedom as a fundamental human right" and condemn the persecution of Christians.

On Saturday night Mr Alexander's words were welcomed by Church leaders. The Rt Rev Lorna Hood, Moderator of the General Assembly of the Church of Scotland, said: "Too often these situations are ignored by politicians worried at offending someone. Such silence should be offensive to all who cherish the right to live safely in a pluralistic society."

The Rt Rev Andrew Proud, the Bishop of Reading, who has previously worked in the Horn of Africa, said: "When leaders in public life have the courage to speak out ... it is a huge encouragement to those who otherwise feel isolated and forgotten. Christians in the Middle East are facing hostility and danger on a regular basis, many losing their homes and their lives because of their faith."