Len McCluskey, General Secretary of the Unite union, addresses delegates on the first day of the Trades Union Congress (TUC) in Bournemouth, southern England, September 8, 2013.
Between May 2010 and December 2014 the Labour party received donations of £48,586,759.96 from trade unions. That figure is just under half of the £110 million that the party took over that period, and illustrates just how important the labour union movement remains Labour's electoral machine.
Since Ed Miliband's general election failure, however, the relationship between the two sides has soured. Now Britain's biggest union, Unite, is set to debate its automatic support for Labour at its July conference with union leader Len McCluskey suggesting that future support from his members would rely on the party picking the "right leader."
The very public spat has already forced prospective Labour leadership candidates to distance themselves from Unite. Even McCluskey's favoured candidate, Andy Burnham, tried to put some clear water between himself and the trade unions by saying that calls for Labour to move to the left "are wrong" and that he is "attracting support from all parts of the party."
Nevertheless, the prospect of Unite dropping its support for Labour will be a big concern for party strategists. Indeed, in many ways the current positioning can be read more as a game of chicken with the party gambling on the lack of viable alternatives for the union to channel its support and funding (worth around £18 million between 2010-2014) towards.
Yet the rift has been a long time in the making.
Although Miliband was elected Labour leader in 2010 on the back of union support, with some unions even including a large picture of him on the ballots that they sent to members, it was his moves while in office to limit their role that marked the beginning of the end of what many had seen as a cosy relationship.
Last year a scandal erupted over the selection of the Labour party candidate for the Falkirk seat. During the selection process allegations were made that Unite had been encouraging its members to sign up as Labour members in order to get its preferred candidate selected.
The claims prompted a review by Labour's National Executive Committee, which found that there was sufficient evidence to warrant concern and placed its own officials in charge of the process. In the aftermath Miliband moved to curtail the close ties between the unions and his party by revoking the ability of trade union members to vote in Labour leadership elections automatically as a result of their automatic union affiliation.
Instead members must now opt in to pay a £3 affiliation fee in order to have their say under the one member, one vote system.