Many MPs returned to Westminster from their constituencies uncharacteristically early today. The cause was the parliamentary boundary review, embargoed copies of which they received at 12pm.

Though the subject is unlikely to quicken voters’ pulses, it is crucial to the outcome of the next general election. The review, which is coupled with a reduction in the number of MPs from 650 to 600, is expected to cost Labour around 30 seats (out of 232) and the Liberal Democrats around four (out of eight). It was for this reason that the two parties combined in the last parliament to delay the review until after the 2015 election.

But empowered by their victory, the Conservatives have resurrected it. Had the alternative boundaries been in place at the last election, they would have won a majority of around 44 as opposed to one of 12.

The planned equalisation of parliamentary constituencies in England and Wales (at plus or minus 5 per cent) will favour the Tories and penalise their opponents. Since Conservative MPs on average represent larger constituencies than their Labour counterparts, they will benefit from the creation of new population-based seats.

As a result of the changes, MPs from the same party will be forced to stand against each other for selection unless one has a superior claim to the new constituency (traditionally based on whether it contains at least 40 per cent of their former seat). Jeremy Corbyn himself is reportedly among those who could see his existing seat vanish.

Some Corbyn allies hope to use the review to deselect his parliamentary opponents by allowing new candidates to stand in selection contests. It was this stance that Corbyn appeared to endorse when he said recently: "there will be a full and open selection process for every constituency".

But a spokesman subsequently stated that he was referring to existing party rules. "Sitting MPs whose constituencies are not affected would be re-selected through trigger ballots. Jeremy does not support mandatory reselection of MPs." Labour chief whip Rosie Winterton, who is managing the boundary review (to the anger of Corbyn allies), noted: "The national executive committee has already agreed the existing rules and procedure for the selection of MPs in the event of boundary changes". But if, as expected, Corbyn is re-elected by a landslide on 24 September, he will be urged by some allies to overturn this decision. Unite recently voted in favour of mandatory reselection and new NEC member Rhea Wolfson has called for "a much more healthy conversation".

It is not only on the Labour side that the changes could prove divisive. Conservatives set to lose their seats may need to be appeased if Theresa May is to win Commons approval. Before the last election, a Tory MP told me that the victims had been promised peerages or knighthoods. But May, who has signalled her intention to reform the honours system, will be reluctant to deploy such maneouvres.

The final recommendations from the Boundary Commission will not be published until autumn 2018 (one reason, some believe, that the Prime Minister hopes to avoid an early election). If passed by parliament, Labour’s task in 2020 becomes even more forbidding. The party will need to win 106 seats merely to secure a majority of one. By unhappy coincidence, it targeted the same number in 2015 - but with the aim of achieving a majority of 78. The loss of 26 seats at the last election and the Tories’ increased majorities in marginals means that Labour starts from a far worse position than previously. Boundary changes would only add to this electoral Everest.