David Davis has vowed the Government will not “leave one part of the United Kingdom behind”, by allowing Northern Ireland to have a softer Brexit deal.

Answering an emergency question, the Brexit Secretary insisted efforts to avoid a hard border would not see Northern Ireland remain in the EU single market or customs union.

“That is emphatically not something the United Kingdom government is considering,” Mr Davis told MPs.

“We will not be treating one part of the United Kingdom differently from any other part,” he added.

The pledge came as Keir Starmer, Labour’s Brexit spokesman, branded Theresa May’s U-turn on an Irish border deal – after a veto by the Democratic Unionist Party – an “embarrassment”.

“It gives a whole new meaning to the phrase coalition of chaos,” Mr Starmer told Mr Davis.

Nigel Dodds, a DUP MP, attacked the “aggressive and anti-Unionist” Irish Government - warning it would “take a long time” to repair damaged relations with Northern Ireland.

Theresa May's EU deal under threat from DUP over Northern Ireland border dispute

And, in a warning to the Prime Minister, he vowed: “We will not allow any settlement to be agreed that causes a divergence politically, or economically, of Northern Ireland from rest of the United Kingdom

“To do so would not only be politically damaging but economically catastrophic for everyone in Northern Ireland.”

The clashes came after the DUP refused to accept “regulatory alignment” on both sides of the Irish border, as a mechanism to avoid the customs posts and checks that Dublin and Brussels have ruled out.

Mr Davis came under pressure to set out what was meant by the phrase – after Leo Varadkar, the Irish Prime Minister, insisted it meant the same as “no regulatory divergence”.

Dublin’s view appears to be that such a deal would see Northern Ireland effectively remain inside both the single market and customs union.

But Mr Davis insisted there was a crucial difference, which was that “regulatory alignment” would not mean “harmonisation” on trade, or having “exactly the same rules”.

The UK would still be “choosing our own laws”, he argued – a red line for many hardline Brexiteer supporters on the Tory benches.

Pro-EU MPs seized on the confusion to press for the case for the Government to accept the only solution was for the entire UK to remain within the EU’s economic structures.

Labour backbencher Ben Bradshaw urged Mr Davis to grant MPs a free vote on staying in the customs union – a nod to a likely majority for that outcome in the Commons – but he rejected the idea.

Chris Bryant, another former Labour Minister, insisted it was the only way to keep the UK together, while ensuring “we don’t harm our trade”, adding: “Why can’t he just see that?”

But Mr Davis also came under strong pressure to continue to rule out any “half in half out solution” – which he was happy to do.