Centre-stage in Derek Simpson's grand suite of offices in Covent Garden, central London, within earshot of the Royal Opera House, is a giant horseshoe-shaped table where the Unite boss holds court. It is identical to the one in Gordon Brown's "war room" at 12 Downing Street, where, in June, the Prime Minister and Peter Mandelson battled through the night to stave off the attempted coup led by James Purnell.
Charlie Whelan, Mr Brown's notorious former spin-doctor, now Unite's political director, has a seat at both horseshoes. It is said, in fact, that through Whelan's controlling influence, the leadership of Unite has become little more than an outpost of No 10, with messages passed to Simpson about what Brown wants.
But is this about to come to an end? Is Simpson, a long-term supporter of the Prime Minister, about to pull the plug on Brown to save the Labour Party? Does Britain's biggest union have that power?
It is difficult to find anyone with a nice word to say about the Prime Minister, outside of Sarah Brown's Twitter page, but Simpson, the 64-year-old union warhorse, has been loyal until now.
Yet on the eve of what is almost certain to be Brown's final conference season as Prime Minister, the Unite joint general secretary cannot summon up a ringing endorsement. "I am as satisfied with Gordon as I can be with any of them. I don't see anybody else that could have done as well, and there are many who would have done a lot worse." He insists Brown is the "right person to lead the party" but adds: "Gordon is very often shuffling and hesitant... and Gordon's not always the best communicator. Following Blair for a start... one thing you could never criticise Blair for was that he could spin a yarn and tell a story."
Under Brown, he says, Labour has "failed to keep in touch with its core vote". But he blames the policies – part-privatisation of the Post Office, the failure to introduce a windfall tax against energy companies – rather than the Prime Minister personally.
Government policies are "half-measures" and "blandishments" which are "not done with any great enthusiasm or passion" – such as the car scrappage scheme, which Simpson says was introduced late.
He is scathing about Brown's key policy document, shaped by Peter Mandelson, "Building Britain's Future". "Of course we need to gear Britain for the future, but the election happens to be in eight months' time, not eight years' time.
"We want a change of party. If the people in the party cannot change the policy, we need to change the people in the party. If he [Brown] could come with the policies that I talk about, in the way I talk about them, not only will he have a chance, he will win.
"He has been undermined and beset by elements in the Labour Party who want to go down the path to disaster." Simpson says that not only is New Labour dead but that old Labour is "lost" and may have been "destroyed".
Unite gave the Labour Party £15m last year, and Simpson apparently hopes his words will act as a gun to Brown's head. "What are the consequences of us not giving Labour money? That will really impair, fatally damage, any chance of Labour winning a general election. We give money to allow the Labour Party to function."
On Friday, Simpson delivered a similar message to Brown when he was one of 15 general secretaries who trooped into Chequers ahead of the TUC conference, which starts in Liverpool tomorrow.
Unite, with 2 million members, is the most powerful of the union section of Labour's electoral college. Whenever a Labour leadership contest comes, whoever receives Unite's backing would be a favourite to be Brown's successor.
Rather than back Ed Balls, long seen as the Brownite torch-carrier, Simpson says that "if Gordon fell under a bus", then the next leader should be Ed Miliband, Secretary of State for Energy. "I think he's quite good, if I had to pick a leader for the future. It's clear that there's some middle ground in the party that's got to be won and it's clear that the middle ground wins it."
He claims this middle ground is not occupied by leading Blairites David Miliband, James Purnell or Peter Mandelson. Using very uncomradely language, the union chief describes these three and other "right-wingers" in the Labour Party as "thick" and "Tories" who would lead Labour to defeat. "Some of them are a bit thick, but are not that thick that they are going to take the leadership of the party to a massacre."
Instead, Simpson claims, the Blairite tendency has decided "it's better to have a Tory government than a proper Labour government, because some of them are Tories".
The Prime Minister must show "that he is prepared to move aside people of the New Labour approach".
"Mandelson's extremely capable, there's no two ways about that. But I don't think we needed him. The way we're going, he [Brown] is going to be the Prime Minister whose party is slaughtered. The only way he can win is to adopt policies that mean standing up to Mandelson, taking on the right of the party. He's nothing to lose because he's going to lose anyway."
What of Mandelson's old enemy? Whelan, who was earlier this year accused of bullying by three Unite officials, which he denies, is no doubt fuelling this anti-Mandelson message. Hasn't Whelan turned Unite's top table into an outpost of No 10? Simpson denies this, and claims the relationship with Brown is "quite useful" to the union, rather than the other way round. "It helps us to have access to the Prime Minister."
Unite, lacking the harmony that its name suggests, is dogged by internal dispute. Simpson was re-elected to the Amicus wing of the union after a bitter battle earlier this year, while there is friction between him and Tony Woodley, who heads the wing of the former TGWU. Next year there will be a fresh battle as the union tries to elect a single general secretary.
Simpson says he and Woodley are "good friends", but adds: "There are clearly differences in how the unions are run that causes irritations from time to time.
"There is obviously a concern on all sides about who's going to be the next general secretary – whether the union looks a little bit more like the T&G or a little more like Amicus. You can't ever say everything's sweetness and light, but on the serious issues, the answer is, yes, we do run a united union."
Simpson refuses to talk to Richard Balfe, David Cameron's envoy to the TUC and a member of Unite. He believes Cameron is a "dupe" for the same old Tories, and that after the election the "claws will come out". But he admits that the Tory leader is "more appealing than Gordon".
Simpson says the UK should make a planned withdrawal from Afghanistan, describing it as a "waste of life, money and time". Not for the first time during the interview, Simpson tries to make a joke – in this case saying he "wouldn't go to Afghanistan for a holiday – then again I wouldn't go to Birmingham for that matter".
His manner remains jovial until I ask him about his stay in a £399-a-night suite at the Waldorf in London for a four-day union meeting, when his union-funded home in Hertfordshire was a 35-minute train ride away. It is the last question, and his response is tinged with an air of menace: "I've stayed in much less salubrious surroundings.
"My past experience is that posh hotels that we have regular bookings with, we get discounted rooms and upgrades, because we're regular customers. So when I was actually told the money that we were paying, I thought even with the discount it was exorbitant, and I've no longer stayed there." In Liverpool, he is staying at the Atlantic Thistle. "Put that in. I've stayed at Travelodge as well as the Waldorf."
For a union leader whose 2 million members who will be more familiar with Travelodge, talk of upgrades at the country's top hotels seems startlingly out of touch. Perhaps for all his tough words and threats, it is Simpson who is a man on his way out.