Theresa May has told EU leaders that it took “comprises on both sides” to reach the Brexit agreement struck on Monday, ahead of the 27 prime ministers and presidents formally signing off the deal on Friday.

Speaking over dinner in Brussels on Thursday evening the PM told the other national leaders that she hoped the accord on the transition period would create a “new dynamic” in talks and that it would now be possible to come up with a solution to the Northern Ireland border issue.

The Monday agreement saw the UK drop its opposition to implementing new EU rules during the transition period, as well as accepting full free movement rights for citizens who arrive before January 2021. The EU, for its part, softened its language on allowing the UK to negotiate free trade deals with other countries during the 21 months.

“Monday saw our negotiating teams take another decisive step towards the smooth and orderly withdrawal of the UK from the European Union by agreeing the terms of the implementation period and large parts of the withdrawal agreement,” Ms May told the leaders over dinner.

In an address in which she also briefed leaders on the Salisbury poisoning, the PM said the Brexit deal took “compromise on both sides” but that it delivered “certainty” for both citizens and business in the EU and the UK.

She said that “following Monday’s agreement we have the chance, now, to create a new dynamic in the talks, to work together to explore workable solutions – in Northern Ireland, in our future security cooperation, and in order to ensure the future prosperity of all our people”.

The PM added: “This is an opportunity it is our duty to take and to enter into with energy and ambition.”

UK government officials say Britain will now try and solve the Northern Ireland border with the so-called “Option A” – building a trade relationship with the entire EU that removes the need for border checks. They say that the EU’s reluctance to talk about the future relationship made it impossible to solve this issue until now.

EU chief negotiator Michel Barnier has however previously said Britain has ruled out “Option A” by ruling itself out of the Customs Union and Single Market, the PM’s red lines. Mr Barnier has said the European Commission is awaiting “Option B” – a specific solution applicable only to the island of Ireland – or that Britain will have to use the “backstop” option of full regulatory alignment.

The latter backstop is fraught with political obstacles because it would effectively require customs checks between Northern Ireland and Great Britain, a state of affairs the DUP would not support. The Government relies on DUP votes for its majority in the House of Commons.

One senior UK government official speaking in the margins of the Brussels summit said: “Once we move onto the stage of being able to talk about the future partnership, that is where we believe the solution to the Irish border will be found. It’ll be a solution in terms of our entire relationship with the EU.

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“Obviously we haven’t been able to have those solutions in the first phase of talks, we’re now moving onto the stage where we can.”

The Northern Ireland issue was effectively parked in the conclusions of the most recent round of talks, with no significant progress beyond the PM reiterating commitments she made in December.