Brexit: Your basic questions answered What you need to know about Brexit.

LONDON -- The U.K. Parliament is once again engaging in a round of debate over Prime Minister Theresa May’s Brexit plan.

In December, the House of Commons held five days of debate over the agreement secured by May and European officials that covered the terms of the British withdrawal from the European Union -- but when it looked certain that her plan would fail to get passed by MPs, she postponed the vote.

The U.K. voted in June 2016 to leave the EU by a majority of 52 to 48 percent. But the question on the ballot did not specify what relationship Britons wanted with the bloc, and so the last two years have been fraught with negotiation and politicking.

May’s premiership has taken a battering, although she survived a vote of no confidence in December after rebels in her Conservative Party plotted to overthrow her through a party mechanism for triggering a leadership contest. They failed, but in order to get MPs to back her, May promised she would not fight the next general election as leader.

But things are far from resolved in Westminster. A previous vote in the Commons means that Parliament must have a vote on the deal with the EU. Having postponed it in December, with hopes that spending Christmas with constituents and having more time to reflect might mean that MPs would back her deal, May is now fighting once again for Parliament to accept her plan.

What is "Brexit"?

On June 23, 2016, Britain voted to leave the EU in a referendum delivered by the ruling Conservative Party. Then-Prime Minister David Cameron resigned the morning after the vote, and May won Conservative leadership to begin implementing the withdrawal.

The U.K. voted to join the EU in 1973 when it was known as the European Economic Community. It is now a group of 28 countries governed by a continental parliament, commission and council representing each member state; it uses a common currency (the Euro) and compromises an area across the European continent where citizens of member states are able to live and work easily in different areas within the union.

What are the main issues?

The Brexit referendum asked a simple question: Should the U.K. leave the EU, or remain in the union?

But there is conflict over whether Britain should retain some aspects of European membership, if it is able, in order to preserve much of its close trade and security relationship with the E.U.

What’s the issue with May’s deal?

MPs are not happy with parts of the deal, which cover aspects of the border with Northern Ireland. A clause in the agreement would mean that the U.K. would remain in a customs arrangement with the E.U. if a permanent solution was not found within the negotiating period.

Doing this would avoid the need for a "hard border" of security and customs checks between Northern Ireland and the Republic of Ireland, which is in the EU. An open border is of paramount importance to both sides of the Irish, and its settlement brought peace after the conflict known as “The Troubles.”

The clause, known as "the backstop," is a red line with which many in the U.K. on both sides of the Brexit debate vehemently disagree. The Europeans maintain their stance that they will not renegotiate May’s deal.

What happens if May loses the vote?

The vote on the deal is scheduled for Tuesday, and as things stand the government appears to be heading for a heavy defeat. If May loses the vote, what happens afterward will largely depends on how badly, and how some political heavy hitters decided to vote.

There is speculation that if May suffers only a narrow defeat, she might make an emergency trip to Brussels for a last-ditch attempt at compromise. Or she may have cross-party talks to try to come to a compromise on the British side.

What most of Westminster fears, however, is the U.K. exiting the EU without a deal -- which would mean a withdrawal falling back on World Trade Organization rules. Business consortiums have more or less presented a united front against such an outcome, saying it would severely harm Britain’s economy.

Another option would be for May to buy extra time by pausing what is known as "Article 50" of the Treaty of the European Union -- the mechanism for beginning an exit from the EU.

When triggered, that begins a period of two years of preparation before an exit date. May invoked it on March 29, 2017, meaning the day of Brexit is set for that same date in 2019.