Cabinet ministers are reportedly openly planning to oust British Prime Minister Theresa May over her failure to secure a Brexit deal.

The Sunday Times is reporting that 11 unidentified, senior British cabinet ministers have agreed to ditch the PM, as the deadline for the United Kingdom to leave the European Union looms within a week.

The speculation follows a petition that gathered more than 4.3 million signatories in three days and mass protests in London on Saturday (British time) in which more than one million people took to the streets to cancel Brexit.

If Ms May is toppled, Brexit would be thrust into doubt. It is unclear how, when and even if Britain will leave the EU.

Ms May, who voted to stay in the EU and won the top job in the chaos following the 2016 referendum, had vowed to deliver Brexit.

But she undermined her prime ministership with a botched snap election in 2017 that cost her party its parliamentary majority.

🚨BREAKING: A full blown cabinet coup is under way tonight to remove Theresa May as prime minister 🚨

— Tim Shipman (@ShippersUnbound) March 23, 2019

The Brexit divorce deal she struck with the EU in November has been overwhelmingly rejected twice by British MPs.

The Sunday Times cited 11 unidentified ministers who said they agreed that the Prime Minister should stand down, warning that she has become a toxic and erratic figure whose judgement has “gone haywire”.

“The end is nigh. She will be gone in 10 days,” The Sunday Times quoted an unidentified minister as saying.

“Her judgement has started to go haywire. You can’t be a member of the cabinet who just puts your head in the sand,” the newspaper cited a second unidentified minister as saying.

The Sunday Times reported that Ms May’s de-facto deputy, David Lidington, is one contender to be interim prime minister but others are pushing for Environment Secretary Michael Gove or Foreign Secretary Jeremy Hunt.

The newspaper said cabinet ministers would confront Ms May on Monday. If she refuses to go, ministers would threaten to resign.

Ms May’s office declined to comment on the reports.

Earlier a Downing Street source, speaking on condition of anonymity, told Reuters that a Times report of discussions in Ms May’s office about her departure was incorrect.

Betting odds indicate there is now a 20 per cent chance that Ms May will be out of her job by the end of this month, Ladbrokes said on Saturday.

Brexit had been due to happen on March 29 before Ms May secured a delay in talks with the European Union on Thursday.

Now a May 22 departure date will apply if Parliament rallies behind the British Prime Minister this week and she is able to pass her deal.

If she fails to do so, Britain will have until April 12 to offer a new plan or decide to leave the European Union without a treaty.

-with AAP