Constitutional specialist Vernon Bogdanor wrote recently in the New Statesman
The Labour Party is composed of three main elements – the Parliamentary Labour Party, the trade unions and the members. But the PLP is the most important, given that it represents the nine million people who voted Labour in 2015, and any future Labour government will be responsible to MPs. A government is not, ought not, and cannot be constitutionally responsible to the few hundred thousand party members outside parliament, who represent nobody but themselves and who are, in Labour’s case, apparently, three times more likely to be well-off urban professionals than the population as a whole. (Emphasis added)