British Prime Minister Theresa May has pushed back against calls for a second Brexit referendum, warning it would mean unpicking the deal agreed with Brussels.

Appearing before senior MPs, Mrs May refused to be drawn on what would happen if the Commons votes down the Withdrawal Agreement in the crunch vote on 11 December.

However, she insisted a so-called "people's vote" was not an option as it could not be held before 29 March 2019 when Britain leaves the EU.

Her warning came as the BBC confirmed that Mrs May had agreed to take part in a televised Brexit debate on Sunday 9 December - two days before the Commons vote.

Mrs May told the Commons Liaison Committee that seeking an extension to the Article 50 withdrawal process - to enable a referendum to be held - would mean the agreement would fall and they would have to go back to the negotiating table.

"Any second referendum that would be held, if that were the case, would not be able to be held by 29 March next year. You would have to extend Article 50," she said.

"To extend Article 50, actually you are then in the business of renegotiating the deal.

"What is clear is that any extension to Article 50 - anything like that - reopens the negotiations, reopens the deal. At that point, frankly, the deal can go in any direction.

"We would simply find ourselves in a period of more uncertainty, more division in this country."

Her warning came amid intense speculation that the British government is heading for defeat in the vote on 11 December, with scores of Tory MPs declaring publicly that they intend to oppose the deal.

With Labour and the other opposition parties also opposed to the agreement, shadow chancellor John McDonnell has suggested a second referendum could be "inevitable" unless Mrs May goes back to the country in a general election.

Mrs May acknowledged that there was a series of "practical steps" which would have to follow if the Government lost the vote but refused to be drawn further.

"My focus is on the vote that will take place on December 11 here in this House," she said.

"You want to look at all sorts of options and ideas. I think it is important Members of Parliament focus on the nature of this vote.

"This is an important point in our history. It is a vote on which we will be deciding whether we deliver on the decision of the British people.

"What has been made clear from the European Union is that this is the deal that has been negotiated and this is the deal that people need to focus on when they are looking at the vote."

In Brussels, the EU's chief negotiator Michel Barnier said the time for negotiations was over and that the British Parliament needed to decide whether to ratify the agreement.

"Given the difficult circumstances of this negotiation, and given the extreme complexity of all the subjects related to the UK's withdrawal, the deal that is on the table ... this deal is the only and the best deal possible," he said in an address to the European Parliament.

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It comes as DUP leader Arlene Foster earlier called on Mrs May to "not waste time" on her Brexit deal and instead look for a better solution to the Irish border.

Reiterating that her party's ten MPs would not support the prime minister's Withdrawal Agreement when it comes to Parliament, she said it would create a "huge democratic deficit" in Northern Ireland.

Speaking on BBC Radio 4's Today programme, Mrs Foster said the current proposals contain "68 pages of regulations that will apply to Northern Ireland and will not apply to the rest of the UK".

"There is a huge democratic deficit coming our way if we agree to this deal, because we've no say over the rules that will apply to Northern Ireland," she added.

Mrs Foster said Theresa May's Brexit deal would see Northern Ireland "diverge off from the United Kingdom" as she called on her to "look for a better deal" that "takes back control of of our money, of our borders and of our laws".

The DUP leader said: "All the things that made us vote for Brexit are the things that are going to be imposed on Northern Ireland."

She also hinted that if Mrs May's current Withdrawal Agreement fails to get through the Commons, her party could be open to a so-called "Norway-plus" style arrangement, where the whole of the UK stays in the customs union.

Mrs Foster says the DUP's "one red line" is to make sure Northern Ireland is not differentiated from the rest of the UK in terms of customs and is "not prescriptive" about other potential options on the future relationship with the EU. 

Meanwhile, the Governor of the Bank of England has warned that large parts of the British economy are not ready for a "no-deal" Brexit.

Mark Carney told BBC Radio 4's Today programme that "less than half of businesses" have put contingency plans in place for such a scenario.

He said: "All the industries, all the infrastructure of the country, are they all ready at this point in time?

"And, as best as we can tell, the answer is no."

It comes after the bank released analysis yesterday warning Britain would be tipped into a recession worse than the financial crisis in the event of exiting the EU on 29 March with no agreement in place.