Blair's worst-kept secret is out: I'm a pretty religious kinda guy

Religious faith was "hugely important" to Tony Blair's premiership, the former prime minister has revealed in an interview to be broadcast today. But he said he was reluctant to discuss it while in office for fear voters would see him as "a nutter".

Mr Blair's former communications chief, Alastair Campbell, once famously told journalists "we don't do God" when asked about the ex-PM's beliefs. But Mr Campbell now says that Mr Blair "does do God in quite a big way", asking aides to find a church he can attend every Sunday, wherever he is in the world.

Another close confidant, Peter Mandelson, said: "He's not an exhibitionist when it comes to religion but deep inside him it is very, very important. This is a man who takes a Bible with him wherever he goes and last thing at night he will read from the Bible."

In an interview for BBC1's The Blair Years, the former Prime Minister made clear how central his faith was to his work at 10 Downing Street. "There is no point in me denying it, I happen to have religious conviction. I don't actually think there is anything wrong in having religious conviction – on the contrary, I think it is a strength for people."

He added: "To do this, the prime minister's job, properly you need to be able to separate yourself somewhat from the magnitude of the consequences of the decisions you are taking the whole time – which doesn't mean to say, and let me emphasise this, that you're insensitive to the magnitude of those consequences or that you don't feel them deeply. If you don't have that strength it's difficult to do the job, which is why the job is as much about character and temperament as it is about anything else. For me having faith was an important part of being able to do that."

Mr Campbell's refusal to discuss Blair's faith while in office was because "you always get into trouble talking about it," said Mr Blair. While in the US and some other countries, it is considered natural for politicians to be open about their faith, "you talk about it in our system and, frankly, people do think you're a nutter," said Mr Blair. Voters tended to think that religious politicians "go off and sit in the corner and... commune with the man upstairs and then come back and say 'Right, I've been told the answer and that's it'," he said.

Mr Campbell told the programme that Mr Blair did not appear to be "your classic religious sort of guy... because he's pretty irreverent, he swears a fair bit and... if he sees a very attractive woman his eye will wander and all that stuff," said Mr Campbell. But he added: "His close circle always understood that there was a part of him that was really, really important – just in the logistical level, wherever you were in the world on a Sunday you had to find a church. On that kind of spiritual level it did inform a lot of what he talked about, what he read... what he felt was important."

Mr Campbell said the British public were "wary of politicians who go on about God". He had always steered clear of the subject fearing the Tories would claim that Labour was trying to claim religious belief as its own territory.

Former Liberal Democrat leader Sir Menzies Campbell suggested that voters may not have backed Mr Blair so strongly in three general elections had they known how closely tied up his political work was with his religious belief. "The public might have been less willing to give him the triumph of three consecutive general election victories if they'd known the extent to which ethical values would overshadow pragmatism," said Sir Menzies.