THERESA May has been given just 10 days by Brussels to make further concessions on Brexit as the prospect of a snap General Election in Ireland threatens to disrupt the negotiation process further.

The clock is now ticking down to December 4 when the Prime Minister is due to have dinner with Jean-Claude Juncker, the European Commission President, which looks set to be the decisive moment on whether or not the EU27 will agree or not that sufficient progress has been made to move onto talking about trade and transition. It will come just days before the next European Council.

After talks with Mrs May at an EU summit, Donald Tusk, the European Council President, said it was "possible" sufficient progress could be made at the crunch December summit to take the talks into the second phase on trade and transition but admitted it remained a "huge challenge".

"We need to see progress from UK within 10 days on all issues, including on Ireland," he declared.

The issue of the border with the Irish Republic is proving a fraught one as is that of the divorce bill.

The Cabinet this week agreed to upping the initial offer of £18 billion with unconfirmed suggestions Mrs May and her colleagues were willing to double it to push the talks onto the second phase. But UK ministers are adamant any increase is contingent on Brussels moving the talks onto trade and transition.

However, EU sources have made clear the EU27 will “refuse point-blank” to offer any guarantees on trade until Britain has met its financial obligations by coming up with a firm figure.

As she left the gathering of European leaders in Brussels, Mrs May said: "There are still issues across the various matters we are negotiating on to be resolved but there has been a very positive atmosphere in the talks and a genuine feeling that we want to move forward together."

The UK Government will now spend the next 10 days in a major diplomatic push to get the EU27 on board. As well as talks with Mr Tusk, the PM had a one-to-one with Angela Merkel, the German Chancellor, as well as meetings with Lars Lokke Rasmussen, the Danish premier, Charles Michel, the Belgian Prime Minister, and Saulius Skvernelis, her Lithuanian counterpart, in the margins of the Eastern Partnership summit.

On the border issue, Mrs May is coming under intense pressure from Dublin for fresh assurances there will be no "hard border" between Northern Ireland and the Republic with Taoiseach Leo Varadkar warning that deadlock in Brexit negotiations could not be broken until the issue is resolved.

However, the threat of a general election in Ireland, which could topple the minority Fine Gael administration, is adding to tensions in the Brexit talks.

On Friday, Mr Varadkar met Fianna Fail leader Micheal Martin for under an hour in Dublin amid a deepening row over attempts to have a senior minister sacked.

Mr Martin, whose party is keeping the Fine Gael Government alive through an 18-month-old confidence and supply agreement, has refused to budge on a motion of no confidence in Frances Fitzgerald, the deputy prime minister.

The vote is scheduled for next Tuesday and has been called over the handling of a 2015 email that revealed attempts to discredit a Garda whistleblower.

In Brussels, Mrs May stressed how London and Dublin had the “same desire" on the border issue, explaining: "We want to ensure that movement of people and trade across that border can carry on as now."

Her comments came after Downing Street backed away from suggestions that Northern Ireland's continued membership of the EU customs union could be up for negotiation in Brexit talks.

A Number 10 spokesman told reporters the UK continued to look for "an innovative way forward" on the issue but when asked if Northern Ireland could remain in the customs union following Brexit, he replied: "That is a matter for negotiations."

Later, a Downing Street source made clear the Government's position that the whole of the UK would leave both the customs union and single market after Brexit had not changed.