FILE PHOTO: British and European Union flags are seen next to Christmas tree before arrival of British Prime Minister Theresa May to meet European Council President Donald Tusk, at the EU Council headquarters in Brussels, Belgium December 11, 2018. REUTERS/Yves Herman

BRUSSELS (Reuters) - European Union leaders gave Prime Minister Theresa May assurances on Thursday that they would seek to agree a new deal with Britain by 2021 so that the contentious Irish “backstop” is never triggered.

Ireland blocked a line that had been included in an earlier draft of the EU statement promising Britain that the bloc would look into giving May more assurances later on.

“It is not open for renegotiation,” the 27 national leaders said in a joint statement of the Brexit deal they agreed with May in late November. The British leader has since returned to Brussels to ask for assurances she says she needs to get it passed by her divided parliament.

The bloc said it wanted to “establish as close as possible a partnership with the United Kingdom in the future” and would aim to have it in place by the end of 2020 “so that the backstop will not need to be triggered.”

If the backstop were triggered, the EU said it would apply “temporarily, unless and until it is superseded by a subsequent agreement” that would ensure no return to a hard border on the island of Ireland.

The bloc promised to undertake its “best endeavors” to ensure the backstop, a fall-back guarantee to avoid a hard border, would only be applied “for as long as strictly necessary.”