IT WAS a “tough day,” admitted a Downing Street insider with a deal of understatement as Theresa May hit the telephones to save the Brexit talks and almost certainly her premiership.

Things were perhaps made a little more testing in No. 10 as the Prime Minister’s frenetic telephone diplomacy took place amid the jovial backdrop of the staff’s Christmas party.

“The No 10 staff Christmas party was going on around the PM,” admitted her spokesman with the emphasis on “around”.

There were phone-calls to Eurocrat-in-chief Jean-Claude Juncker, Taoiseach Leo Varadkar, the DUP’s Arlene Foster and Sinn Fein’s Michelle O’Neill.

It was the two phone-calls to Mrs Foster as the night wore on which were the most crucial given it was the DUP leader’s dramatic veto on Monday when the talks plunged into crisis.

The first inkling that things were moving in the right direction came around 8pm when Donald Tusk, President of the European Council of leaders, let it be known that he would make a statement as dawn broke on Friday; quite significantly, before the financial markets opened.

Yet the talks between London, Brussels, Belfast and Dublin were still not completed.

“It was a long night,” admitted the PM’s spokesman. “The text kept changing until it was in a place everyone felt what we needed to achieve, protecting the integrity of the UK single market, had been achieved.”

At 11pm after Mrs May spoke again to Mrs Foster, relief began to spread around Downing Street as it was at this stage that the DUP confirmed it was finally on board.

The relief prompted Julian Smith, the Government chief whip – identified by some as the man who had wrongly assured the Prime Minister that the DUP had agreed to Whitehall’s initial proposal – to post a tweet. “Theresa has worked tirelessly this week to try to move EU negotiations onto the next stage in the National Interest,” he declared.

To underline the mood change, he even tweeted a picture of a smiling Mrs May in conversation with none other than the Brexiteer-in-chief Boris Johnson, who had popped into No 10 earlier in the day.

Just before 1am, the Prime Minister caught up on some sleep; just two hours before she was up again to prepare for her quick flight to Brussels. Leaving No 10 at 3.30 am, she boarded the military flight at RAF Northolt near Oxford at 4.30am.

At 4.57am, No 10 announced Mrs May was in the air, en route to the Berlaymont building in the Belgian capital for croissants and orange juice with Mr Juncker and the EU’s chief negotiator Michel Barnier.

As she arrived in Brussels just after 6am, the earlier mood of anguish and anxiety had clearly lifted.

Martin Selmayr, Mr Juncker’s chief aide, signalled a deal had been done by tweeting a photograph of white smoke gushing from the chimney of the Sistine Chapel; the traditional way of alerting the world that a new Pope had been chosen.

Despite her lack of sleep, the PM looked refreshed and very relieved as she attended a joint press conference with Mr Juncker to announce the breakthrough. It was in stark contrast to their glum appearance on Monday, which lasted barely two minutes.

By 9.30am Mrs May was back in Britain and en route to the reassuring comfort of her Berkshire constituency after a week like no other.

She will now spend the weekend concentrating on her Commons victory statement about the Brexit breakthrough. Expect the Tories to raise the roof.