"We don't do God," former Downing Street spin doctor Alastair Campbell is reputed to have said when his boss, the then-Prime Minister Tony Blair, was asked about his faith. Plenty of politicians do though, including it now transpires, current Prime Minister David Cameron. Here's why it is not always an easy mix.
Tony Blair, who was received into the Roman Catholic faith after leaving office, has always been a man of deep Christian faith. He could never quite understand why he could not share this with the voters. On one occasion, he wanted to end a prime ministerial speech with the line "God bless Britain", but was persuaded out of it by aides. "One of the civil servants said in a very po-faced way 'I just remind you prime minister, this is not America' in this very disapproving tone, so I gave up the idea. I think it is a shame that you can't since it is obviously part of what you are," he recalled, in 2012.
Cameron does God
David Cameron used to belong firmly to the "don't do God" tradition, once saying his faith came and went like "the reception for Magic FM in the Chilterns" (a line borrowed from Boris Johnson). But he has now decided to tap into a different tradition, speaking about he is a regular churchgoer, and urging Britain to be "more confident about our status as a Christian country".
The Tory party at prayer
The Anglican faith used to be synonymous with the Conservative Party. It was known as the Tory Party at prayer. But since the 1980s, when bishops began to speak out against Margaret Thatcher's policies, the two institutions have had a more prickly relationship. The latest flashpoint has been gay marriage.
UKIP if you want to
Some see David Cameron's decision to speak more openly about his Christian faith as an attempt to heal some of the divisions caused by his support for gay marriage. Nigel Farage has accused him of trying to "mimic" UKIP to woo back disaffected Tory traditionalists. Although the UKIP leader has never been an evangelical Christian. "I think there is something there, but that's as far as it goes," he told The Guardian when asked about God.
Faith of the faithless
In the UK, politicians have always worn their religious faith, or lack of it, lightly. Two out of the three main party leaders are atheist. Nick Clegg "came out" as a non-believer shortly after being elected Lib Dem leader in 2007, although his wife is Catholic and his children are being brought up in the faith. Labour leader Ed Miliband, pictured here with wife Justine on a recent trip to Israel, had Jewish parents but was brought up in a Marxist home. "I am not religious but I am Jewish," he says.
Disraeli the trailblazer
If he wins next year's general election, Ed Miliband will be only the second prime minister of Jewish birth. The first, Benjamin Disraeli, was one of the founding fathers of the modern Conservative Party. Born to Italian-Jewish parents, in 1817 Disraeli's father baptised his children as Christians. With Jews excluded from parliament until 1858, this enabled Disraeli to pursue a career that would otherwise have been denied him.
Labour at prayer
The Labour left used to take its cue from Karl Marx, viewing religion as the "opiate of the masses" but there has always been a strong strand of Methodism in the party, connected, in part, to the temperance movement. Tony Benn was very much in that tradition. Tony Blair - like his friend and mentor John Smith, who he replaced as Labour leader - belonged to a more recent tradition, the Christian Socialist movement. Gordon Brown often spoke of how he got his "moral compass" from his father, a Church of Scotland minister. But they were the exceptions rather than the rule in the secular world of British politics.
The last atheist
It is a very different story in the US, where "God Bless America" is the traditional presidential sign-off. American politicians are not only expected to have faith, they are expected to talk about it. They could not get elected otherwise. Until he lost his seat in 2012, Californian Democrat Pete Stark used to be only self-declared atheist in Congress. Now there are none.
'God bless America'
Not all US presidents have been ostentatious about their faith, but all have been practising Christians, with most belonging to churches, from the Episcopalian George Washington through a succession of Unitarians, Quakers (Richard Nixon) Methodists (George W Bush), Presbyterians (Ronald Reagan) to Southern Baptists like Jimmy Carter and Bill Clinton. John F Kennedy was the first, and so far, only Catholic to occupy the White House. There has never been a Jewish, or a Muslim, president. Although a 2012 poll by the Pew Centre, suggested 17% of Americans thought Barack Obama was a Muslim. Obama is a practising Christian, and is often seen, as in the picture above, heading to a small Baptist church in Washington DC on Sunday, with his family.