David Davis's resignation has put Theresa May's future in doubt

• David Davis resigns as Brexit Secretary, throwing Theresa May's government into crisis.
• His resignation letter accuses May of betraying the result of the Brexit referendum.
• Davis's departure has thrown May's future into doubt.
• the prime minister will tonight address Conservative MPs as the threat of a leadership challenge mounts.

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LONDON - David Davis's resignation as Brexit Secretary just months before the UK has to conclude Brexit negotiations, is a crushing blow to Theresa May's authority as prime minister and has put the entire future of her government in doubt.

His resignation letter is brutal, effectively accusing May of betraying the result of the Brexit referendum by handing back control of Britain's future to the EU.

"In my view the inevitable consequence of the proposed policies will be to make the supposed control by Parliament illusory rather than real," he writes.

"As I said at Cabinet, the "common rule book" policy hands control of large swathes of our economy to the EU and is certainly not returning control of our laws in any real sense."

So why did David Davis resign, what does it mean for the future of Brexit negotiations and can May now survive?

Here's everything you need to know about how things could play out over the coming hours and days.

May's darkest hour

David Davis has long been unhappy with the direction May's government has been taking on Brexit and his increasingly sidelined position.

Although repeatedly threatened, Davis held back from resigning under the understanding that May would not water down her 'red lines' of leaving the EU customs union and single market.

The Cabinet agreement on Friday to effectively stay in the single market for goods, and therefore water down any possibility of meaningful trade deals after Brexit, changed all that.

"At each stage I have accepted collective responsibility because it is part of my task to find workable compromises, and because I considered it was still possible to deliver on the mandate of the referendum, and on our manifesto commitment to leave the Customs Union and the Single Market," Davis writes.

"I am afraid that I think the current trend of policy and tactics is making that look less and less likely."

Most damningly of all, Davis suggests that May could shift even further towards a soft Brexit.

"I am also unpersuaded that our negotiating approach will not just lead to further demands for concessions," he writes.

From the moment she became prime minister, May has insisted that "Brexit means Brexit," and denied all attempts to push Britain towards a softer interpretation of the referendum result.

By questioning May's willingness and ability to deliver such a hard Brexit, outside of the single market and customs union, Davis has shot an arrow through the heart of her premiership, and it could well prove to be fatal.

Will there now be a leadership challenge against May?

May will this evening address Conservative MPs in a private meeting inside parliament. These meetings are normally an opportunity for the prime minister to rally her troops and rarely result in any great dissent from the backbenches.

Tonight's meeting could be very different. Tf May fails to stabilise her party tonight then there is, for the first time, a real risk that Tory MPs will decide that enough is enough.

In order to trigger a challenge, MPs need to lodge a letter with the chair of the so-called 1922 committee of backbench MPs. If more than 48 do so, and close to that number have reportedly already done so, then a leadership challenge will be triggered.

For the first time that now looks like a very real possibility.

Is May's future on the brink?

While there may be at least 48 Conservative MPs willing to trigger a challenge against May, it is highly unlikely that such a challenge would ultimately be successful.

The vast majority of Tory MPs, although far from happy with the current state of May's government, are not yet convinced that the solution is to replace the prime minister.

Not only is there no obvious single successor for May, a challenge now, just months before Brexit negotiations need to conclude, would be an act of enormous self-harm for both the party, the government and the country.

More importantly for May's most anti-EU critics in the party, getting rid of May now would not fundamentally change the parliamentary arithmetic, which is now firmly in favour of a softer Brexit.

Even if the prime minister were to be replaced by a more passionately Leave-supporting minister, there is no parliamentary majority for the sort of no-deal Brexit being pushed by May's Brexiteer critics.

Are we heading for a general election?

Any attempt to replace May now would also put huge pressure on the government to call a general election, which would not only risk putting Jeremy Corbyn in power, but also threaten the whole future of the Brexit project.

Whatever MPs think of May and her Brexit strategy there are very few among their number who face the prospect of another general election with anything but horror.

For this reason, any challenge against May would almost certainly fail, leaving her in power but significantly weakened, just when her authority is most needed to secure a successful outcome from Brexit negotiations.

This would be the worst of all worlds for May's party and most sensible Conservative MPs will do everything they can to avoid it.

For this reason May is likely to survive, at least in the short term, whether there is a challenge against her or not.

However, it is now very clear that she is on borrowed time. Conservative MPs have repeatedly told Business Insider that there is likely to be a challenge against her once Brexit negotiations conclude and Britain has left the EU next March.

It is now a question of when, not if, May is forced out before the next general election.

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