(CNN) British Prime Minister Theresa May told lawmakers Monday that while the vast majority of a Brexit deal has been reached, there remains a "considerable" sticking point with how to deal with the border between Northern Ireland and the Republic of Ireland.

However, May says she remains hopeful the issues can be resolved before the end of the transition period.

"I see any extension... as being undesirable. By far, the best outcome for the UK, for Ireland, and for the EU, is that our future relationship is agreed and in place by the first of Jan 2021," the Prime Minister told the House of Commons.

"I have every confidence that it will be."

However, she added: "There are some limited circumstances in which it could be argued that an extension to the implementation period might be preferable if we were certain it was only for a short time.

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