LONDON  —  Prime Minister Theresa May will on Monday convene her cabinet and update parliament on her newly-agreed Brexit deal, as the embattled British leader tries to sell the maligned plan to a sceptical country.

May returns to a mutinous Westminster after sealg the agreement with European Union leaders at a summit on Sunday in Brussels, where both sides insisted this was the best and only option available.

But she faces a big battle to win MPs’ approval ahead of a vote next month, with lawmakers from all parties -- including her own Conservatives and governing allies the Democratic Unionist Party (DUP) -- opposed.

"We can back this deal, deliver on the vote of the referendum and move on to building a brighter future," May will tell the House of Commons, according to excerpts of her planned remarks released by Downing Street.

"Or this house can choose to reject this deal and go back to square one.

"It would open the door to more division and more uncertainty, with all the risks that will entail," she will warn lawmakers.

‘National interest’

The agreement sealed Sunday prepares for Britain’s smooth exit from the EU on March 29, 2019, and sets out a vision for "as close as possible a partnership" afterwards.

It covers financial matters, citizens’ rights, provisions to keep open Britain’s border with Ireland and arrangements for a 21-month post-Brexit transition phase.

The lengthy, legally enforceable divorce deal is accompanied by a short political declaration setting out hopes for future ties, including security, trade and migration.

Until it is approved by parliaments in Britain and Strasbourg, all sides are still planning for the potentially disastrous possibility of Britain quitting the European Union with no new arrangements in place.

A European source told AFP that May herself acknowledged in Brussels on Sunday that she did not currently have a majority in parliament to pass her plan.

The prime minister vowed to warn her rebellious MPs that at least half could lose their seats at the next election if they fail to deliver her Brexit deal, the source said.

She will now embark on an intensive nationwide campaign promoting the plan -- and the dangers of no-deal -- ahead of the vote by lawmakers on or around December 12, according to reports.

"I believe our national interest is clear," she will tell MPs on Monday.

"The British people want us to get on with a deal that honours the referendum and allows us to come together again as a country whichever way we voted."

Challenging arithmetic                                                                                   

But arch-Brexiteers despise the agreement, arguing it keeps Britain too closely aligned with the bloc.

Alongside the DUP, they also dislike the so-called "backstop" plan for Northern Ireland after a 21-month post-Brexit transition period.

If used, it would keep the British-ruled province more closely aligned with the EU in certain areas so as to avoid a hard border with remaining EU member Ireland.

The Northern Irish DUP party, whose 10 MP’s prop up May’s government, has pledged to "review" its support if her plan gets approved by parliament.

Meanwhile more than 80 Tory MPs have declared publicly that they intend to vote against it.

"I don’t believe that, so far, this deal delivers on what the British people really voted for," former Conservative leader Iain Duncan Smith told Sky News on Sunday.

Cabinet colleagues have conceded it was an uphill struggle but remain resolute.

"The arithmetic at the moment is looking challenging but a lot can change over the next two weeks," Foreign Minister Jeremy Hunt told the BBC on Sunday.

"We’re going to get through this and give people the Brexit they voted for," he added optimistically.

However opponents were also readying for the fight.

Labour leader Jeremy Corbyn said his party will oppose the package in parliament and work with MPs from other parties to block a no-deal Brexit.

"This is a bad deal for the country," he said in a statement.

"It is the result of a miserable failure of negotiation that leaves us with the worst of all worlds." — AFP