Andy McSmith: The reality of Unite could not be further from Tory myths

"Your party is a wholly owned subsidiary of the Unite union," David Cameron told Gordon Brown in a furious exchange at yesterday's Prime Minister's Questions. "They pick the candidates, choose the policies; they elect the leader; they have special access to Downing Street."

Moments later, the Liberal Democrat leader Nick Clegg added his ha'penny worth by suggesting that Unite is to the Labour Party what Lord Ashcroft, the billionaire non-dom, is to the Conservative Party.

Unite is Labour's biggest backer, having donated £11m since 2007, about a quarter of the party's income. It has a large contingent of delegates on Labour's National Policy Forum, and wields a block vote at annual conferences that is almost as big as the votes of all the constituency Labour parties.

Gordon Brown's former spin-doctor, Charlie Whelan, is in and out of the Commons looking after the interests of his former boss while working as political director of Unite. A Unite national officer, Clare Moody, works full time in the Prime Minister's political office – her salary paid by the union.

According to the union's website, 160 Labour MPs and 25 peers are members of Unite. Dozens more Unite members are among the new Labour candidates in seats which the party hopes to hold at the general election. They include Harriet Harman's husband, Jack Dromey, Unite's deputy general secretary, who has secured the safe seat of Birmingham Erdington.

The latest controversy to blow up this week concerns the fact that a Unite official, Peter Wheeler, has been shortlisted for Stalybridge and Hyde – the safe Labour seat in Lancashire vacated by the Blairite ex-cabinet minister, James Purnell. Mr Purnell's former adviser, Johnny Reynolds, seen by some as the natural successor, was excluded from the shortlist.

But it is a curious myth that Unite is some kind of monolith controlling the Labour Party and pulling it to the left. It is, arguably, one of the most misnamed organisations in the land, being anything but united. Formed by an unhappy marriage in 2008 between the TGWU union and Amicus, it has two head offices and two general secretaries, Tony Woodley and Derek Simpson, who barely speak to one another.

Amicus was an amalgam of several unions that were pillars of the right wing of the Labour Party and the TUC. The last time that the Stalybridge constituency was up for grabs, before the 2001 general election, Tony Blair personally rang Amicus's general secretary, Sir Ken Jackson, to ask him to help get the nomination for James Purnell. Mr Purnell was then co-opted on to Amicus's parliamentary panel.

Amicus helped out again after Peter Mandelson's friend Shaun Woodward, the former Tory MP for Witney, switched parties and needed a safe Labour berth. Sir Ken helped parachute him into St Helens South.

Ten years ago, Amicus's forerunner, the manufacturing workers' union the AEEU, was thrown into head-on conflict with the TGWU and came close to being expelled from the TUC. The cause of the row, ironically, was Bassa, the union for British Airways cabin crew – the people whose decision to go on strike this weekend has put Unite into the headlines.

About 5,500 Bassa members renounced the union, which was too militant for their taste, and formed a breakaway organisation called Cabin Crew '89, one of whose leaders, Jacqueline Foster, is now a Conservative MEP. As a right-wing union, it naturally teamed up with the AEEU, while the more militant Bassa linked itself to the TGWU, setting off a furious row between the two big unions. For years, leaders of the two factions refused to speak to each other until they were formally reunited by the Amicus-TGWU merger.

These old battle scars have not gone away. If Charlie Whelan, whose background is in Amicus, rang up Bassa's militant shop stewards and told them to call off a strike that is threatening to embarrass the Labour government, he would be told in short order to get lost.

The strangest myth of all about Unite is the one repeated yesterday by David Cameron – that it has the power to choose the next leader of the Labour Party. In Labour leadership elections, union members vote individually, by post. There is no evidence that the mass of Unite members, or members of any other union, turn to their union leaders for advice on how to vote.

In the 1994 leadership contest, Tony Blair swept up the union votes, although most of the heads of the big unions would have prevented him becoming leader, if only they had known how. In the more recent contest for Labour's deputy leadership, in 2007, Unite's leaders backed the Dagenham MP, Jon Cruddas. He came third.