The fraught parliamentary maths for Theresa May on Brexit

Theresa May is facing the daunting task of winning parliamentary approval for her compromise Brexit deal after EU leaders signed off on the terms of the UK’s departure from the bloc.

The Conservatives lost their majority in the House of Commons at the 2017 general election, and Mrs May relies on the support of the Democratic Unionist party’s 10 MPs to push key legislation through parliament.

But the DUP has vowed to oppose Mrs May’s Brexit deal, as have Eurosceptic and Europhile Conservatives, so the prime minister risks losing her promised “meaningful vote” on the withdrawal agreement — unless she convinces scores of MPs to change their minds or abstain. The vote is due to take place on December 11.

How the Brexit votes stack up

There are 650 MPs*. Mrs May in theory needs 320 votes in the Commons to secure approval of her Brexit deal, because the speaker John Bercow and his three deputies do not vote, and seven Sinn Fein MPs have decided not to take up their seats.

The solid circles below show the approximate minimum number of MPs in each group. The hollow circles represent additional MPs that could take the group to its maximum size**.

May’s core support

Conservative loyalists
Between 225 and 290 MPs

About 150 Conservative MPs are on the government’s payroll: ministers and aides known as parliamentary private secretaries, and they will be expected to vote for Mrs May’s Brexit deal if they want to keep their jobs. Plus there are backbench Tory MPs who are loyal to their party and do not want to see a no-deal Brexit, a second referendum or a general election.

Labour rebels
Between 5 and 15 MPs

A handful of hardcore pro-Brexit Labour MPs such as John Mann are likely to support Mrs May’s deal. Some other Labour MPs — including Remainers such as Caroline Flint — could also back the withdrawal agreement because a majority of their constituents support Brexit.

Democratic Unionist party
Between 0 and 10 MPs

Arlene Foster, DUP leader, has vowed that her party’s 10 MPs will vote against Mrs May’s Brexit deal because it treats Northern Ireland differently to the rest of the UK. However, Downing Street is hoping the risk of a Labour government led by Jeremy Corbyn, who has longstanding links to Irish republicans, may prompt the DUP to change its stance.

These groups of MPs above give Mrs May a best case of 315 votes for her Brexit deal. While this falls short of the 320 votes the prime minister needs to be assured of victory, she may have other routes to success.

Large-scale abstentions by MPs who would otherwise oppose her deal would allow Mrs May to win with substantially fewer than 320 votes. The Conservative party machine will be seeking to persuade, cajole and pressure not just moderate Eurosceptic Tory MPs but also hardcore Brexiters to back or at least not oppose the deal.

The Brexit deal sceptics

Conservative Remain rebels
Between 5 and 10 MPs

At least five pro-European Tory MPs are expected to vote against Mrs May’s deal if it fails to keep the UK close to the EU customs union and single market. Should the deal be rejected by parliament, these MPs are likely to call for a second referendum.

Conservative Leave rebels
Between 20 and 80 MPs

The pro-Brexit European Research Group, led by Jacob Rees-Mogg, counts about 80 Conservative MPs among its supporters. These hardcore Tory Brexiters could reject Mrs May’s deal, although some may back her because of concern about the risk of a general election and Labour seizing power.

Mrs May will find it hard to secure support from the groups of MPs below: most are expected to vote against her deal, although some could abstain.

The opposition parties

Labour loyalists
Between 240 and 250 MPs

Mr Corbyn has signalled Labour MPs will vote against Mrs May’s deal because it will have failed the party’s six tests on Brexit. But by rejecting the withdrawal agreement he could anger a substantial minority of Labour supporters who back Brexit.

Smaller parties
Up to 62 MPs

The Scottish National party’s 35 MPs are expected to vote against Mrs May’s deal. Most of the Liberal Democrats’ 12 MPs have pledged to reject any deal that is based on her compromise Brexit plan. Plaid Cymru’s four MPs and the Green party’s one are also expected to vote against. The DUP has also repeatedly said its 10 MPs will vote against.

* Includes independent MPs.

** All numbers are estimates.

Letter in response to this article:

Royal assent denied to customs union in 12th century Frankish East / From Charles Mercey, Tellisford, Somerset, UK