European Commission president Jean-Claude Juncker arrives to address the European Parliament during a debate on upcoming EU council meeting, at the European Parliament in Strasbourg, France, December 11, 2018. REUTERS/Vincent Kessler

STRASBOURG (Reuters) - The European Union is willing to give Britain further clarifications on its Brexit deal but will not renegotiate the treaty or its protocol on the Irish border, EU chief executive Jean-Claude Juncker said on Tuesday.

Addressing the European Parliament in Strasbourg, the European Commission president said he was “astonished” at Prime Minister Theresa May’s inability to get the package agreed with the EU last month through the British parliament.

Noting that he would meet May in Brussels on Tuesday evening ahead of an EU summit on Thursday and Friday, Juncker said: “The deal we achieved is the best possible. It’s the only deal possible. There is no room whatsoever for renegotiation.”

“But of course there is room enough to give further clarifications and further interpretations without opening the withdrawal agreement,” he said. “The withdrawal agreement will not be reopened.”

Juncker repeated that neither side wanted ever to use a “backstop” that would keep Britain in a customs union with the EU in the absence of a better way to avoid extensive border checks between Northern Ireland and EU member Ireland.

“We have a common determination to do everything to be not in a situation one day to use that backstop but we have to prepare. It’s necessary for the entire coherence of what we have agreed. It’s necessary for Britain and it’s necessary for Ireland. Ireland will never be left alone.”