There are few programmes that divide Radio 4's loyal listeners as sharply as Thought for the Day. At around ten to eight in the morning, half of the Today programme's audience is grateful to begin the day with this brief oasis of calm. For the other half, it's time to jump out of bed and head for the shower.
"I love it. I'd be lost without it," says Sandi Toksvig, the comedian and Seven magazine columnist. "It gets me up in the morning. I wake up, turn over and turn it off. I'd find it difficult to be without it."
The broadcasting equivalent of Marmite, it has been a fixture on the current affairs programme for 40 years and is one of the station's untouchable slots.
But that could all change as the BBC Trust is due to rule on whether Thought for the Day should be broadened to include the voices of secularists. It follows a series of complaints that by not doing so the BBC is failing to fulfil the obligations in its charter.
The "God slot" can count the Prince of Wales among its fans but, despite having supporters in the highest of places, the clamour has grown in recent months for it to change its policy of exclusivity – or be dropped altogether.
And with a decision expected within the next few weeks, the behind-the-scenes battle between secularists and believers has intensified.
Senior Church of England bishops have privately lobbied the trust over the importance of maintaining the status quo. Meanwhile, The Sunday Telegraph has learnt that secularists have warned the corporation it would be in breach of equality laws if it refuses to make the slot more inclusive. Lawyers have been asked by senior management at the BBC to investigate the claim so that they can advise the trust on whether they would be at risk of facing the fight going to court. This legal challenge is a new twist in a battle between secularists and believers - and neither side is prepared to lose.
Thought for the Day may be a mere three-minute slot, but it has become a totemic issue in the social struggle over the role of religion in public life.
There is already a much wider representation of religious views than when it started in 1970. In 1992, Umar Hegedus was the first Muslim invited on to the slot; Jews, Sikhs and Hindus have also joined as the cultural make-up of Britain has evolved.
Secularists argue that barring their voices is indefensible as the country has changed so dramatically from the society that Lord Reith, the BBC's founding father, was addressing when he placed religion at the heart of public service broadcasting – a time when there were nearly twice as many churchgoers as there are now.
"Thought for the Day is a relic of the past," believes Claire Rayner, the agony aunt who is vice-president of the British Humanist Association. "It's a slot that encourages disparity. Let it die quietly."
With the rise of secularism, critics of the slot's policy are irritated that religious leaders are free to express their views unchallenged to an audience of 4.5 million.
"Thought for the Day is a rebuke to the BBC guidelines that promise balance and a fair hearing for everyone," says Terry Sanderson, president of the National Secular Society. "It is the only contentious programme on radio where the speakers face no challenge. If it can't be opened to a wider range of voices, it should be taken off the air."
This is already happening on local radio, with a growing number of regional BBC stations allowing secularists to make contributions to their own Thought for the Day-style slots. BBC Radio Kent announced last week that it was dropping the religious slot from its breakfast programme altogether, prompting the Rt Rev Stephen Venner, the Bishop of Dover, to complain that Christians were being marginalised by the station.
In 2002, Radio 4 did air an atheist's Thought for the Day. Richard Dawkins was invited to broadcast one when Rod Liddle, then producer of the Today programme, bowed to calls for it to be more reflective of the listenership. In response, more than 100 public figures, including the playwright Harold Pinter and author Iain Banks, sent letters to the BBC governors – now the Trust – demanding the ban on non-religious contributors be lifted. One BBC insider told The Sunday Telegraph that Liddle was only too glad to give their cause some publicity as he wanted the slot scrapped.
The Rev Giles Fraser, canon chancellor of St Paul's Cathedral, believes the slot's survival is all the more remarkable given that it has been under constant attack. "That it has managed to survive for so long is because it appeals to a very British desire for a gentle and non-threatening sense of the divine," he says.
Esther Rantzen, the broadcaster, says this is exactly why she is so fond of it. "I think the clattering conveyor-belt of news threatens to deafen us all. Life is so pressured these days that just to stop for a moment should be a gift."
Its emphasis on reflection is one of its main failings, according to Liddle, who says this results in it being woolly and insipid. "Thought for the Day is secular already," he says. "God is almost never allowed to poke His nose into a broadcast, and when He does, His appearance is heralded with apologies and embarrassment. He does no smiting, He is never angry."
Jon Snow, the Channel 4 news presenter, agrees. "It's like the weather forecast. It's just there."
His comments have been echoed by other critics in the past, leading the BBC to send some of the contributors on voice-coaching sessions in an attempt to make their delivery less monotonous. And the occasionally bland nature of the content has been blamed on the forensic scrutiny that producers apply to each of the scripts, checking that there is nothing likely to cause offence.
This has not prevented controversy from slipping through. Anne Atkins provoked complaints after she suggested that people from Norfolk are not very clever. And when Lavinia Byrne, a Catholic nun, was dropped from the slot she complained that it was "rapidly becoming Opinion for the Day rather than Thought for the Day".
One BBC insider suggested that people are particularly sensitive about Thought for the Day because it is so familiar to them. "Radio 4 listeners see the station as their own and they don't like change."
At a debate last week at St Peter's in Marylebone, London, Canon Fraser said the calls for secularists to be included on the slot was akin to putting cricket on Match of the Day.
The secularists responded that their exclusion would mean the BBC thinks religious people have a monopoly on morality and ethics.
The trust is in an unenviable position. And if the drawn vote at last week's debate is any indication, the future of Thought for the Day hangs very much in the balance.