Theresa May given the weekend to change her Brexit plans or risk a Cabinet walkout.

A senior Cabinet source tells Business Insider that May faces a "killer moment" in Brexit talks.

Cabinet Brexiteers are angry with May's refusal to put a definite end-point on her plans for a so-called Brexit "backstop."

At least three ministers are poised to quit next week unless May changes course.

Under the planned backstop, the United Kingdom would effectively remain within the Customs Union indefinitely after Brexit, in order to prevent a hard-border with Northern Ireland.

LONDON — Theresa May has this coming weekend to change her Brexit plans or suffer a series of walkouts from her Cabinet, government sources have told Business Insider.

Leave-supporting ministers Andrea Leadsom, Penny Mordaunt, and Esther McVey are believed to be ready to quit the Cabinet if May agrees to a backstop which keeps the UK in the customs union indefinitely.

A senior Cabinet source told BI that the House of Commons leader Andrea Leadsom will decide on Monday whether to resign once the prime minister has presented her final backstop proposal, with her Cabinet colleagues including Mordaunt and McVey also poised to quit.

McVey, the Secretary of State for Work and Pensions, refused to declare her support for May's Brexit plans on Thursday after International Development Secretary Mordaunt refused to do so earlier in the week.

"This isn't just one more little compromise from the prime minister to get a deal over the line... it is the litmus test for the taking back control... this is the killer moment," a Cabinet source told BI.

"There will be a flurry of calls over the weekend between ministers," they said. "These days ministers rarely say what they mean in Cabinet because everything gets leaked... The prime minister needs to hear the mood by Monday."

The prime minister needs to hear the mood by Monday.

They added that until now, pro-Brexit ministers have been reluctant to quit, despite reservations with recent developments in Brexit talks, because they feel it will be easier to alter May's plans from inside Cabinet.

"What good are Boris [Johnson] and [David] Davis doing shouting from the outside? We need Brexiteers in the Cabinet," they said.

However, with just days to go until May's crucial summit with EU leaders, ministers believe action must be taken now.

They added that for the prime minister to wait until Tuesday's Cabinet meeting and somehow "bounce" ministers into supporting her deal would be "the worst possible way of doing it."

The back-stops here

Penny Mordaunt Jack Taylor/Getty Images

The latest Cabinet row is over the terms of the Irish backstop, which is the final issue to be resolved in Brexit negotiations between UK and EU negotiators for there to be a deal before the March deadline.

The backstop is the insurance policy for making sure the frictionless border between Northern Ireland and the Republic is maintained after Brexit. It will come into effect at the end of the proposed transition period, in January 2021, unless a new trading arrangement which protects the open Irish border is in place.

The current plan is for the UK to remain in the customs union during this period while Northern Ireland alone would also remain fully aligned to the single market. Cabinet Brexiteers are just about willing to accept this, as long as the backstop comes with a fixed endpoint — for example: it would cease on on January 29, 2023.

A spokesperson for the prime minister refused to commit to a time-limited backstop on Friday morning and instead said May would pursue a "temporary" arrangement.

May is also facing a rebellion from DUP MPs who prop up her government over new checks between Northern Ireland and the rest of the UK which come with the backstop plan currently being negotiated in Brussels.