LONDON — Forget Brexit and the historic realignment of the U.K.’s place in the world. As of next week there will be a new topic raising the blood pressure of British MPs: the biggest redrawing of Britain’s electoral map since the 1920s.

From noon on Monday, MPs will be visiting the Boothroyd Room in the heart of the Westminster village. There they will receive, under condition of confidentiality, copies of a proposed new political map with 50 fewer parliamentary seats.

The plans for England and Wales, which will be made public Tuesday (proposals for Scotland will follow in October, while Northern Ireland’s were released earlier this week), are expected to put a bomb under the opposition Labour party: It is projected to lose 25 to 30 of its seats and see up to 200 affected by boundary shifts, according to an independent analysis by the Conservative peer and political reform specialist Robert Hayward.

The cull will disproportionately hit Labour seats in Wales and the north of England, at a time when the party is languishing in the polls and riven by infighting. If the plans do win parliamentary approval — a vote is due in 2018 — it would be a coup for new Prime Minister Theresa May, smoothing her passage to a second term. But with many Conservative MPs worried about the future of their own seats and disgruntled that as the House of Commons takes a cut, the unelected House of Lords continues to grow, May could face an uphill struggle to push the reforms over the line.

The proposals will also trigger a months-long process of horse-trading that will pit MPs of all parties against their colleagues.

“Every MP will be looking anxiously for their seats," a Labour source said. "For those affected, this is literally going to take over people’s lives.”

The Conservatives are set to lose between 10 and 15 seats, with the other losses shared between the Scottish National Party and smaller parties, according to Lord Hayward's independent analysis, which has circulated around Westminster in recent days, and appears to confirm Labour's fears of a big electoral hit.

The proposals will also trigger a months-long process of horse-trading that will pit MPs of all parties against their colleagues, as parliamentarians whose seats are going to disappear vie for somewhere to stand in the 2020 general election — like a giant game of musical chairs.

With 50 seats cut, it will be the biggest reduction in the number of U.K. MPs since Ireland became independent in 1922.

Size matters

Originally ordered by former Prime Minister David Cameron in 2011, the cull is ostensibly aimed at leveling out variations in population sizes between constituencies to ensure each has between 71,000 and 78,500 registered voters. Currently, they vary in size from less than 41,000 in Arfon, Wales, to more than 110,000 on the Isle of Wight (the Scottish Western Isles, and Orkney and Shetland islands, with their tiny populations, are outliers that will be excluded from the review).

The rationale behind the cut, according to Lord Hayward, is that the U.K. has "overlarge legislatures" for a Western democracy. The government argues it will make every vote more equal in value, while cutting the cost of parliament by £66 million every five years.

The number of constituencies has fluctuated in the past, but cutting 50 MPs is a radical step that Labour MPs — and, privately, some Tories — believe is little more than a Tory plot to tighten their grip on the House of Commons.

The final map, drawn up by the independent Boundary Commissions, remains under wraps until Tuesday. While the judge-led commissions from England, Scotland, Wales and Northern Ireland have been praised by all parties for the way they have undertaken the review, the decision to cut 50 MPs rather than just redrawing existing boundaries is set in stone by legislation passed by the last Tory-led government. In other words, the commissions' hands are tied.

Labour anticipate that the plan could bear resemblance to 2013’s aborted Boundary Review, which was blocked by the Conservatives' then coalition partners the Liberal Democrats, who voted alongside Labour. Then, the seats of senior figures, including the new Brexit minister David Davis, then a backbencher, and the new International Development Minister Priti Patel, were put at risk.

But it is Labour who have the greatest cause for concern this time. On top of the threat to their number of seats, with the majority of the party’s MPs in revolt against leader Jeremy Corbyn, there are fears that grassroots "Corbynistas" will use the boundary changes as an excuse to oust his critics.

The constituencies of former Shadow Chancellor Chris Leslie, and prominent figures including London MPs Stella Creasy and Mike Gapes, could see significant reshaping. Under Labour rules, local constituency party members would choose the new parliamentary candidates in cases of significantly re-shaped constituencies, with standing MPs potentially having to seek re-nomination against hard-left candidates.

But any such moves are still a long way off. The proposals published on Tuesday will undergo a 12-week public consultation during which the parties will have a chance to air their grievances. Another round of consultation will follow before revised plans are published in autumn next year, with final ratification by parliament by the fall of 2018.

Peer pressure

Already, however, there are signs that the plans could meet the same fate as their 2013 predecessor. Labour are fiercely opposed, the SNP also have concerns, and a number of Conservatives have already broken ranks — despite the obvious political gain awaiting them in 2020; a prize Theresa May will not want to relinquish.

“It does seem completely perverse and an utter nonsense to reduce the size of the House of Commons by 50. That's 50 people who are elected, while we continue to pump people into the unelected House of Lords,” said the Conservative Chair of the House of Commons Procedures Committee Charles Walker.

Disquiet among MPs over the rapidly expanding House of Lords — the unelected chamber has gained 205 new peers since 2010 — peaked when it was revealed over the summer that Cameron had created 16 new peers in his resignation honors list.

One Conservative MP said the move had caused “significant irritation” and could hinder May’s attempts to ensure all her MPs get behind the boundary changes. Some Tory MPs are also expected to push for the number of ministers to be cut proportionate to the number of MPs.

Whips of all parties are also expected to face a difficult task over the coming months ensuring favored parliamentary candidates are able to enter the House of Commons at the next election — a process that one Conservative figure said would involve persuading older MPs that the next election might be the right time to retire.

Another senior Tory, former minister Ed Vaizey, said that while there would be some “hard cases” for the Tory whips, he expected the plans to pass a Commons vote in 2018.

“I see why it would concern colleagues, if your constituency is going to be broken up," he said. "But a lot of us do recognize that there are huge disparities between populations in what are largely Tory seats and those in some largely Labour seats. It’s an anomaly that has to be corrected.”

Others are not so supportive. The Electoral Reform Society campaign group opposes the decision to base the constituency map on the number of registered voters, rather than local population figures.

“This is a risky and potentially worrying move, given that areas of greatest need and often with the highest proportion of marginalized groups are frequently those with lowest levels of registration,” said campaigner and lawyer Katie Ghose, the group's chief executive. “And while we understand the desire to even out the size of constituencies, fair political boundaries are crucial to ensure our communities are fairly represented in parliament and we shouldn’t tear apart close-knit areas in a rush to equalize numbers.”

The government, however, insists that the shake-up is necessary and senior Conservative figures remain convinced that, with careful people management, enough potential rebels can be persuaded to back the reforms. One senior Conservative suggested that, with a retirement rate of 35 MPs in each five-year parliament, the party would have a far easier time freeing up constituency seats for MPs left high and dry than the Labour party will.

“As it stands some constituencies have twice as many electors than other constituencies and this cannot be right,” said Cabinet Office Minister Chris Skidmore. “These reforms will ensure fair and equal representation for the voting public across the United Kingdom by the next general election.”