DUBLIN (Reuters) - The European Union’s chief Brexit negotiator Michael Barnier sought to reassure Ireland on Thursday that their interests are shared in divorce talks with Britain and that he will work with Dublin to avoid a hard border returning to the island.

With close trading links to Britain and the only land border with the United Kingdom, Ireland is widely considered the remaining European Union member with the most to lose when its bigger neighbor quits the bloc.

“I am fully aware that some member states will be more affected than others,” Michel Barnier said in an address to both houses of the Irish parliament, an honor usually only reserved for visiting heads of state.

“I want to reassure the Irish people: in this negotiation Ireland’s interest will be the Union’s interest...

“Brexit changes the external borders of the EU. I will work with you to avoid a hard border. Nothing in this negotiation should put peace at risk,” Barnier said.

Irish government ministers and officials have held over 400 meetings with European counterparts since last June’s referendum in Britain to stress the need to avoid the return of a physical barrier or customs controls along the border with Northern Ireland.

The frontier was marked by military checkpoints until a 1998 peace deal ended three decades of violence between Catholic nationalists seeking a united Ireland and Protestant unionists who wanted to keep Northern Ireland British. Over 3,600 people died.

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Irish Prime Minister Enda Kenny has said any semblance of a return to what could be deemed a hard border could lead to a renewal of the armed conflict. But Brussels, Dublin and London have yet to clearly explain how the currently invisible border can be maintained after Brexit.

In negotiating guidelines agreed last month, EU leaders said the unique circumstances on the island of Ireland will require flexible and imaginative solutions. These included the aim of avoiding a hard border.

Barnier, who praised Ireland’s “remarkable preparatory work” on Brexit and quoted Nobel Prize winning Irish poet Seamus Heaney to lawmakers, warned that Brexit will come at a cost to all members and that customs controls are part of the EU’s border management to protect the single market.

Kenny said on Thursday that British Prime Minister Theresa May needed to give greater clarity on how she expects to retain the closest possible relationship with the EU after Brexit.

Barnier, upon whom Dublin has focused much of its diplomatic offensive, will see the issues first hand on Friday when he travels with Ireland’s Foreign Minister to visit some food businesses and farmers who work along the border.