Jeremy Corbyn has been accused of “gerrymandering” by blocking reforms to an electoral system that is biased in favour of Labour and has become “an affront to democracy”.

New analysis shows that 27 million voters are being under-represented by their MPs because they live in constituencies where the MP has a higher than average number of constituents, making it harder for each voter to get their voice heard.

Constituency boundaries are more out of date than at any time since the Second World War, but Labour is opposing changes that would equalise constituencies in a slimmed-down Parliament of 600 MPs, 50 fewer than at present.

The majority of the larger constituencies are in Conservative voting areas, which has led to claims that the system is biased against the Tories.

Chris Skidmore, the Minister for the Constitution, has accused Jeremy Corbyn of “political opportunism” in trying to block reforms, saying that any future general election result would be “tainted” without the necessary changes being made.