Theresa May is in the fight of her political life as she battles to get her Brexit plan approved by her Cabinet and then parliament.

Theresa May is in the fight of her political life as she battles to get her Brexit plan approved by her Cabinet and then parliament.

Theresa May has endured a fiery Question Time where she was quizzed about her Brexit plan and is now trying to convince her Cabinet to back it.

Ministers have arrived at Downing Street where the British Prime Minister has called a special meeting of her Cabinet to sell the draft Brexit agreement she and the leaders of the European Union have come up with.

It won't be easy and there is talk she may be forced to quit if the plan isn't given the green light by ministers or if a number of senior figures quit in response.

Even if the plan is approved she faces an uphill task in getting it through the divided House of Commons, where her Conservative Party doesn't have a majority and rebel MPs are expected to cross the floor to vote against the deal.

The BBC reported the prime minister would try and head off the threat of any resignations by telling her ministers that while the agreement was not perfect, it was as good as it could get.

Mrs May had been expected to address media about 5pm local time (4am AEDT), but the meeting had overrun by as much as an hour, and possibly two.

She told the Commons that she would return to the chamber to update them on what happened at Cabinet but that statement is not expected to happen until Thursday.

Before the Cabinet showdown she faced a tense Question Time where MPs - both friend and foe - attacked the plan, and her handling of the negotiations.

Senior conservative MP Peter Bone, who is pro-Brexit, said she was "not delivering the Brexit people voted for" and told her: "Today you will lose the support of many Conservative MPs and millions of voters across the country."

Jacob Rees-Mogg, the leader of the pro-hard Brexit European Research Group of Tory MPs, said the reported deal represented a betrayal of Theresa May's promise to maintain the integrity of the United Kingdom.

"White flags have gone up all over Whitehall. It is a betrayal of the Union," he said.

Labour leader Jeremy Corbyn told parliament "this Government spent two years negotiating a bad deal" and that the Prime Minister is asking MPs to choose between a "half baked deal and no deal".

Mr Corbyn said the PM's deal would result in a "catastrophic series of consequences" and "neither of these options is acceptable".

But Mrs May hit back and declared the Government "will not renege on the decision of the British people" as she insists that "we will take back control of our money, laws and borders".

The main obstacle in negotiations has been how to ensure there are no customs posts or other checks along the border between Northern Ireland, which is part of the UK, and Ireland, which is an EU country.

Britain and the EU agree there must be no barriers that could disrupt businesses and residents on either side of the border and undermine Northern Ireland's hard-won peace process - but they have differed on how to achieve that.

The controversial "Irish backstop" plan is the element of Theresa May's deal which is most likely to cause fury among MPs. The mechanism is designed to ensure there is never a hard border between Northern Ireland and the Republic.

Britain and the EU hope to agree a technological solution to the border problem during trade talks over the next two years.

The backstop is a legally binding arrangement which keeps the border open even if those efforts fail.

Leaked details of Mrs May's deal suggest that it will keep Britain tied to the EU customs union until an alternative solution is found.

The whole UK would continue to follow European rules on customs - and Northern Ireland would stay tied to EU regulations even more closely.

That is likely to concern many in the party as it means Northern Ireland would be treated differently than the rest of the UK - but the backstop would not involve checks on goods travelling between Britain and Northern Ireland.

Demonstrators hold flags outside the Houses of Parliament. Picture: AP

The UK has been denied the right to bring the backstop to an end unilaterally. Instead an independent panel would rule on whether or not talks have broken down, and end the backstop arrangement if so.

Brexiteers in the government are worried that the EU intends to keep the UK in the customs union permanently, using the backstop as cover for that.

All this is further complicated by the fact Mrs May relies on Northern Ireland's Democratic Unionist Party to prop her administration up - a fact not lost on the DUP leader, Arlene Foster, who retweeted this: "Politics may be the art of the possible, but it is also the science of the mathematics of votes."