THERESA May’s future is hanging in the balance after she secured her Cabinet’s blessing for a long-awaited Brexit deal – but was immediately left facing down a mounting Tory rebellion.

The Prime Minister made the historic announcement on the steps of 10 Downing Street following a mammoth five-hour Cabinet meeting during which divisions erupted over the terms of the exit agreement.

Last night, Brexiteer anger exploded into the open as Jacob Rees-Mogg, chair of the influential European Research Group, urged his fellow Tories to ensure the plan is ditched amid claims it would leave the UK tied to EU rules.

In a letter to MPs, he said the deal would see the UK "hand over £39 billion to the EU for little or nothing in return”, and insisted it was “profoundly undemocratic”.

He also raised fears plans to treat Northern Ireland differently to the rest of the UK would break up the Union and fuel the SNP's drive for independence.

The move heightened speculation that Mrs May could face an imminent leadership challenge.

While resignations failed to materialise last night, sources said the Cabinet was deeply divided.

Sky News said ten ministers spoke against the agreement – including Home Secretary Sajid Javid, Foreign Secretary Jeremy Hunt and International Development Secretary Penny Mordaunt.

Work and Pensions Secretary Esther McVey was particularly vocal – and is understood to have even called for a vote over the issue.

Speaking outside Downing Street, Mrs May said: “The choices before us were difficult, particularly in relation to the Northern Ireland backstop.

“But the collective decision of Cabinet was that the Government should agree the draft withdrawal agreement and the outline political declaration.

“This is a decisive step which enables us to move on and finalise the deal in the days ahead.

“These decision were not taken lightly, but I believe it is a decision that is firmly in the national interest.”

The Prime Minister said the choice before the country was clear, and argued her deal delivered on the vote of the referendum and brought back control over money, laws and borders – as well as protecting jobs, security and the Union.

MPs either decided to back it, she said, or risked the UK crashing out of the European Union without a deal.

She added: “I know that there will be difficult days ahead. This is a decision which will come under intense scrutiny and that is entirely as it should be and entirely understandable.

“But the choice was this deal which enables us to take back control and to build a brighter future for our country, or going back to square one, with more division, more uncertainty and a failure to deliver on the referendum.

“It’s my job as Prime Minister to explain the decisions that the Government has taken, and I stand ready to do that – beginning tomorrow with a statement in Parliament.

“But if I might end by just saying this: I believe that what I owe to this country is to take decisions that are in the national interest, and I firmly believe, with my head and my heart, that this is a decision which is in the best interests of our entire United Kingdom.”

Boos and shouts from anti-Brexit campaigners could be heard as Mrs May spoke.

It came after a dramatic day which saw an unprecedented intervention from Scottish Tory MPs over post-Brexit fishing rights.

The 13-strong group insisted the UK “cannot remain in the Common Fisheries Policy” after the end of the transition period.

However Scottish Secretary David Mundell – who was among the signatories and was widely rumoured to be considering his position – later appeared to throw his weight behind Mrs May's deal.

Meanwhile, Brexiteers within the Tory Party ramped up their rhetoric. Earlier in the day, prominent Leaver Peter Bone warned Mrs May she risked losing the support of "many Conservative MPs and millions of voters across the country".

Former Ukip leader Nigel Farage tweeted: "Any cabinet member who is a genuine Brexiteer must now resign or never be trusted again, this is the worst deal in history."

The DUP, which props up Mrs May’s Government, has repeatedly insisted it will not back a deal that treats Northern Ireland differently from the rest of the UK.

Last night, senior DUP MP Sammy Wilson said: "It's a poor deal. It's a bad deal. It's a deal that she said she'd never accept. I think that people will be appalled at this deal.”

News an agreement had been reached between EU and UK officials broke on Tuesday. The full, 585-page text was published last night.

It includes plans for a 21-month transition period after the UK leaves the EU in March 2019, as well as a so-called divorce bill which is believed to be around £39 billion.

But controversy centres on plans for a “backstop” to avoid a hard border between Northern Ireland and the Republic.

Both the UK and EU will "use their best endeavours" to achieve a future trade agreement by July 2020 – six months before the end of the post-Brexit transition period, said the European Commission.

Should this not be possible, the UK and EU could jointly agree to extend the transition period.

Alternatively, they could introduce a "backstop" arrangement from January 2021, establishing a "single EU-UK customs territory" to apply until a new trade deal is implemented, covering all goods except fishery and aquaculture products.

Northern Ireland would remain part of the same customs territory as the rest of the UK and the entire area would be subject to "level playing field" commitments requiring Britain to follow EU rules in areas like animal welfare, environmental and workplace protections.

Michel Barnier, the EU’s chief negotiator, told reporters in Brussels: "In addition Northern Ireland will remain aligned to those rules of the single market that are essential for avoiding a hard border.”

Mr Rees-Mogg said the plans would see different rules for Northern Ireland, adding: “This is unacceptable to Unionists particularly in Northern Ireland, and Scotland where the SNP will seek to demand similar internal UK border to weaken the Union.”

The level of Brexiteer discontent has raised expectations that Mrs May faces further letters of no confidence from Tory MPs, with a total of 48 needed to trigger a vote on her position.

Sources within Westminster said the delivery of letters to the chair of the backbench 1922 Committee Sir Graham Brady was "imminent".

Mrs May described the debate around the famous Cabinet table as "long, detailed and impassioned", in an apparent indication that her proposals had come under intense challenge from ministers.