Labour is backed by 11 trade unions, representing four million workers.
Members of the unions are able to vote in Labour leadership contests, and union representatives sit on the party’s decision-making body, the National Executive Committee.
Unions support Labour financially via membership fees, a proportion of which are given to the party via political funds, as well as dedicated donations.
Labour received £1.4m from seven different unions in donations in the first three months of this year, according to latest figures published last week.
Unison gave £366,936 and Unite, despite having been very critical of the Labour government, donated £392,544.