The European Commission's chief negotiator on Brexit has said that the UK's decision to leave the European Union will have consequences for the EU's external borders.

However, speaking in his first news conference, Michel Barnier said he would do his utmost to "preserve the success of the Good Friday process."

He said he was "personally extremely aware" of the importance of Ireland's situation in the negotiating process.

Once Britain leaves the EU the border between Northern Ireland and the Republic will constitute the EU's external border.

Mr Barnier was asked if he believed there would be a hard border on the island of Ireland.

He replied: "The UK's decision to leave the European Union will have consequences, in particular, perhaps, for what are the EU's external borders today."

Mr Barnier, who has been appointed by the European Commission to lead the negotiations with the UK, recalled that as a one-time regional affairs commissioner he was in charge of the EU's peace programme for Northern Ireland.

"All I can say at this moment in time, is that I am personally extremely aware of the importance of this particular topic.

"We will, throughout these negotiations with the UK and of course with Ireland, do our utmost to find a way in order to preserve the success of the Good Friday process and of course retain the dialogue there.

"I was ... in charge of a very important programme for Northern Ireland [as regional affairs commissioner] and that was the Peace Programme.

"This was in support of the Good Friday Process," he said.

Mr Barnier warned of legally complex and politically sensitive negotiations ahead, and said that there would be only be between 15 and 18 months for the negotiations to be completed once Britain triggers Article 50 of the Lisbon Treaty to formally begin the process of withdrawal.

He said if Article 50 was triggered by the end of March, as British Prime Minister Theresa May has suggested, then withdrawal agreement could be completed by October 2018.

However, he said that the future trading relationship between the UK and the EU would be a separate issue.

It was, he said, up to the UK to spell out what kind of "partnership" it wanted with the EU and for the EU27 to decide on what they would accept.

But he warned that one of the key principles underpinning the EU27's position was that "third countries", ie the UK post-Brexit, could never have the same rights and benefits as members.

He described the EU's four freedoms, including the free movement of people, as "indivisible".

"Cherry-picking is not an option."

Mr Barnier said there would be "some point" in a transitional agreement between the EU and the UK before Britain's future relationship was defined, but, he added, only if it eased the path towards that future "partnership".

He also said that the EU would only decided on the "usefulness" of a transitional agreement when they had a "perspective" on what Britain's long term relationship would be with the EU.

Business groups in the UK and Ireland have warned that a transitional period would be vital to avoid a hard exit from the EU in which companies would only be able to trade with the single market subject to tariffs.

Mr Barnier warned: "President Juncker and I are determined to preserve the unity and the interests of the EU27 in the Brexit negotiations. 

"This determination is shared by all governments."

"Being a member of the European Union comes with rights and benefits. Third countries can never have the same rights and benefits since they are not subject to the obligations."