THERESA May has vowed to protect the "precious Union of the United Kingdom" as she battles to keep her faltering hopes of a Brexit deal alive.

As the Prime Minister prepared to address leaders of the remaining 27 member states in Brussels, she insisted that she remained committed to securing an agreement with the EU.

However, during exchanges at Prime Minister's Questions, Jeremy Corbyn said the Tories were too "weak and divided" to negotiate effectively with Brussels.

Mrs May travels to Brussels for what had been billed as "the moment of truth" in the negotiations amid growing concerns the two sides will be unable to bridge the gap over the key issue of the Northern Ireland border.

In the Commons however she made clear she would not accept any "backstop" arrangement to ensure the border with the Republic remains open which effectively drew a border between Northern Ireland and the rest of the UK.

"We continue to work for a good deal with the European Union - a deal that delivers on the Brexit vote but also a deal that protects jobs and livelihoods and crucially that protects the precious Union of the United Kingdom," she said.

She added: "We will not accept any proposals which would effectively break up the United Kingdom."

Earlier Ireland's deputy prime minister Simon Coveney confirmed the EU's chief negotiator Michel Barnier was prepared to extend the 21-month transition period after Britain leaves the bloc to allow time to negotiate an alternative arrangement.

"What Michel Barnier has indicated very clearly is that the EU side, certainly, is willing to allow more time in the transition period to agree an alternative solution to a backstop," he told the BBC Radio 4 Today programme.

The meeting in Brussels was supposed to be the occasion when the leaders of the EU 27 member states gave the green light for a special summit in November to finalise the terms of Britain's withdrawal.

However European Council president Donald Tusk has warned that without new "concrete proposals" from the British to break the logjam over the Irish border backstop, further progress on a deal may be impossible.

But with her party split, and some Tory MPs openly calling for her to go, Mrs May has little room to manoeuvre if she is to secure a deal which stands any chance of getting through Parliament.

The Prime Minister will briefly address the leaders of the EU 27 on Wednesday evening before they discuss the state of play in the Brexit negotiations over a working dinner while she leaves.

Mrs May will also have bilateral meetings with European Commission president Jean-Claude Juncker, European Council president Donald Tusk, and Irish Taoiseach Leo Varadkar.

Ahead of her visit to Brussels, Mrs May was able to secure the backing of her Cabinet, at least for now, amid reports that some Brexiteer ministers had been prepared to quit if she gave too much ground to Brussels.

However, there was anger among Tory Brexiteers after The Daily Telegraph reported that Chancellor Philip Hammond warned the UK could still have to pay the EU up to £36 billion of the £39 billion "divorce bill" to settle its outstanding liabilities, in the event of a no-deal break.