Much will depend on the result of a crucial vote by MPs in the first week of June on a draft law to implement the Brexit deal which MPs have already rejected three times.

Here are some of the possible scenarios:

The government has said it wants parliament to approve the legislation required to implement Brexit before parliament goes into summer recess on July 20.

The date of the first vote on the bill has not been set but will be in the week starting June 3.

As part of that legislation, the government is promising MPs a vote on whether or not to hold a second referendum.

If MPs approve the law but then vote against holding a second referendum, Brexit would happen at the end of that month under the rules agreed at an EU summit last month.

EU leaders have set the deadline for Brexit on October 31 but have given indications they could accept a further postponement on top of two previous delays.

Brexit was originally supposed to happen on March 29 of this year following a negotiation period after Britain voted by 52 percent in favour of leaving the EU in 2016.

If parliament rejects May's deal for a fourth time, the government may seek yet another delay.

That could allow time for a change of leadership in Britain, a second referendum or a general election or a rethink of whether to go ahead with Brexit at all.

Whether or not EU leaders are open to the idea of a delay could become clear during a Brexit discussion during an EU summit in Brussels on June 20-21.

If there is no further delay, Britain would have to leave the European Union on October 31 with no overall agreement.

This is the scenario advocated by Nigel Farage's Brexit Party and by hardline "Brexiteers" within May's own Conservatives.

But finance minister Philip Hammond is firmly opposed and has warned of his concern over the "significant" short and long-term effects of a no-deal Brexit.

Economists say a no-deal Brexit could generate economic shockwaves on both sides of the Channel, causing severe delays at border points and added costs for business.

Britain has the unilateral right to stop Brexit at any moment by revoking Article 50 -- the formal procedure for member states that want to leave the EU.

Three years after Britain voted to quit the EU, stopping Brexit is seen as a political impossibility.

MPs have already rejected a second referendum in non-binding votes earlier this year.

But, in theory, if MPs do vote in favour of a second referendum on the issue and if the result of that poll is to stay in the EU, Britain would remain.