President of the European Council Donald Tusk has said that there can be no discussions about a trade deal with the United Kingdom until there has been progress in protecting the Northern Ireland peace process.

In a letter to leaders of the remaining 27 European Union countries ahead of a summit tomorrow, Donald Tusk said that "before discussing our future, we must first sort out our past."

Mr Tusk said the negotiations would be based on a phased approach: until EU leaders determined there had been sufficient progress in protecting EU citizens in the UK, on the amount of the UK's financial contribution to the EU budget and on protecting the Good Friday Agreement, there could be no discussion on the future relationship between the UK and the EU.

"This is not only a matter of tactics, but - given the limited time frame we have to conclude the talks - it is the only possible approach," the former Polish leader wrote to the leaders.

"I would like us to unite around this key principle during the upcoming summit, so that it is clear that progress on people, money and Ireland must come first," Mr Tusk said.

"And we have to be ready to defend this logic during the upcoming negotiations."

The Government has sought to have a special text agreed at tomorrow's extraordinary EU summit on Brexit spelling out that if a united Ireland were to come about, then Northern Ireland would automatically re-join the EU.

The text would state that the Good Friday Agreement contains a specific mechanism for a united Ireland, and that if it were to come about, the new entity would legally be a member of the EU.

The move by the Government is highly sensitive, given the tensions surrounding Brexit and the issue of a future border poll in Northern Ireland.

Government sources insist Taoiseach Enda Kenny does not believe it is the right time to trigger such a poll.

However, the Government is determined that all of the elements of the Good Friday Agreement are enshrined in the Brexit negotiation process and understood by the other 26 member states and the EU institutions.

One of the most critical of those elements is the mechanism providing for a united Ireland if a majority of people in Northern Ireland consent.

Mr Kenny has referred on a number of occasions to the German precedent.

When Germany was reunified after the Berlin Wall came down, East Germany automatically became a member of the EU.

Minister for Public Expenditure and Reform Paschal Donohoe has said the Government will seek a "political acknowledgement" to recognise the Good Friday Agreement in the mandate for talks on how Brexit will impact Ireland.

If the EU 27 agree the declaration sought by the Government on Saturday, then it would add political, if not legal weight to such a precedent being followed if there was a referendum on a united Ireland.

The declaration, however, would not be part of the negotiating guidelines.

Instead it would be included in the minutes of the council meeting, and published some time after the summit takes place.

Sinn Féin President Gerry Adams has said the Taoiseach should seek to look for a political declaration in EU guidelines about Brexit that Northern Ireland would get special designated status after the UK leaves the EU.

Mr Adams said he told Mr Kenny prior to this weekend's summit that the Government had a duty to ensure that the wishes of the majority of people in the North who voted to stay in the EU are respected.