British Prime Minister Theresa May said on Sunday that "finish line" of the Brexit talks is in sight, while admitting the last stages of the talks are going to be the hardest of all.

"The hardest part of Brexit is to come but the finish line is in sight, and the deal will be right for you, your family and our country," May said in an article published on the London-based tabloid Sun newspaper.

May called Brexit Britain's "biggest constitutional change for generations," saying that unpacking more than 40 years of treaties, agreements and laws would be a challenge under any circumstances as no country has left the European Union (EU) before.

"With five months to go until we leave the EU, we have already reached agreement on the vast majority of our exit deal," the prime minister wrote in the article, adding that "I am more determined than ever that we will get there together with a Brexit deal that's right."

London and Brussels are working to find a way of inking a future relationship agreement after Britain ends its membership in the bloc.

With a deal between both sides said to be more than 90 percent agreed upon, the question of the border between Northern Ireland and the neighboring Irish Republic has become the big unresolved issue between both sides.