THERESA May is to hold bilateral talks with the Irish premier in Brussels later today to tackle arguably the biggest hurdle to an EU deal – the Irish border issue.

Leo Varadkar, the Taoiseach, is hoping to agree guidelines on the future relationship between the EU and UK at the two-day European Council summit of leaders with the focus on progress in the negotiations.

On Monday, the UK and EU agreed terms for a transition period, which the Prime Minister hopes the EU27 will ratify so that the process of agreeing a trade deal can begin.

The draft text of a withdrawal treaty includes an agreement that there must be a “backstop” solution to ensure there is no hard border on the island of Ireland if no other answers are found.

The Irish Government has insisted the UK has provided a "cast-iron guarantee" that will ensure no physical infrastructure, checks or controls at the border between Northern Ireland and the Republic after withdrawal.

Without any other broader trade solution, Northern Ireland would continue to follow EU regulations which affect the island; effectively staying in the customs union.

Most of the detail around how the border arrangements would operate have not been agreed.

"On Friday, we will meet in Article 50 format to discuss the Brexit negotiations. This is an important moment in the Article 50 negotiations and it is hoped that we can agree guidelines on the future relationship between the EU and the UK,” explained Mr Varadkar.

"I am pleased the UK has now agreed that a backstop solution to avoid a hard border proposed in December will form part of the legal text and that all the issues identified by the EU side in the draft will be addressed to deliver a legally sound solution to avoid a hard border on our island.

"Prime Minister May confirmed this in a letter to President Tusk on Monday."

Discussions over Thursday and Friday will also focus on jobs, growth and competitiveness, external relations, the future of Europe and particularly digital taxation, and further and better economic and monetary union.

The European Council President has said he is recommending EU leaders welcome the transition deal "in practice" as it would "allow to delay all the negative consequences of Brexit by another 21 months".

While a transitional deal is expected to be ratified, at home Mrs May is facing a rebellion from Tory MPs, who have expressed deep unhappiness that Britain will not be taking back control of its own waters on Brexit Day in March 2019 but rather from December 2020.

The PM, who has held private meetings with concerned colleagues, has sought to allay fears that during the negotiations there will not be more concessions to the EU27 on fishing; some MPs, nonetheless, fear that control of UK waters could be “traded off” with Brussels to get a deal on something else such as financial services.

Some Scottish Conservative MPs, who believe the issue of fishing is “iconic” have even threatened to vote against the final deal if full sovereignty over fishing rights is not in it; if the Government were to lose that vote, it would almost certainly see it fall and trigger a general election.