LONDON, Nov. 4 (Xinhua) -- The British government's chief legal adviser has been quietly added as a 12th member of the Cabinet sub-committee designed to oversee London's Brexit negotiations with Brussels after ministers insisted they would not sign off on a deal without his advice.

Geoffrey Cox, the British attorney general and a supporter for the Leave campaign, has been given a permanent seat on British Prime Minister Theresa May's Brexit war cabinet, according to a major British newspaper, the Sunday Telegraph.

The move comes as May is believed to be closing in on a deal with the European Union (EU) over an insurance plan, or "backstop", intended to avoid border infrastructure between Northern Ireland and the Republic of Ireland.

However, Brussels is refusing to agree to an end date or mechanism that would allow the UK to pull out of the arrangement, meaning that an agreement is unlikely to be reached this week.

Senior British government officials have insisted a "get-out clause" is necessary to ensure the country is not left permanently "trapped" in the EU customs union.

Following the 2016 referendum on EU membership, which resulted in a vote to leave the EU, the British government triggered the withdrawal process under Article 50 of the Treaty on European Union on March 29, 2017.

The rapid pace of the negotiations requires the British government to adopt a "war cabinet" approach. It mirrors the approach used by prime ministers of the recent past to make decisions during armed conflicts or incidents like foot and mouth, when normal cabinet committee processes are too cumbersome to keep pace with the demands of decision making.