What next for Brexit?

What happens to the Brexit process now, as prime minister Theresa May fights a challenge to her leadership?

The embattled leader toured European capitals on Tuesday in an attempt to seek concessions on her Brexit deal, less than four months before the March 29 date when Britain is set to leave the European Union.

But the EU has vowed not to budge, and she came home to a potential leadership challenge.

She said she will contest Wednesday's challenge and, if victorious, plans to table a vote on her Brexit deal some time before January 21.

If she loses, the leadership election could take days or even weeks into the New Year and could potentially derail the Brexit timetable.

Here are some of the most probable scenarios:

- Tweaks to the deal -

Victory for May over her rebel MPs will mean she cannot face another no-confidence challenge from within her party for 12 months, giving her some breathing space as she attempts to tweak her deal and get it through parliament.

May held talks with European counterparts on Tuesday seeking "further assurances" over the so-called "backstop plan" for the Irish border as she attempts to drum up support for her deal.

She will meet with EU leaders to discuss Brexit during a summit in Brussels on Thursday.

European sources privately say only clarifications or tweaks in the accompanying declaration on post-Brexit ties might be possible.

But that could be enough to sway MPs, particularly if the extended uncertainty takes a disastrous toll on the economy and financial markets.

- Norway option -

If her deal is rejected, MPs are set to take more control over the whole process. They could push for a "plan B", which would see Britain adopt a softer Brexit, such as staying in the EU's satellite trading bloc the European Economic Area -- the so-called Norway option.

Although being in the single market would require maintaining freedom of movement of EU citizens into Britain -- a contentious issue for pro-Brexit voters -- this approach is considered more likely to command a majority in parliament and potentially pass a vote.

Another potential obstacle, however, is that Britain would have to continue paying large amounts of money into the EU budget, which would prove hugely unpopular.

- No-deal Brexit -

Britain has legislated to leave the European Union on March 29, 2019, after triggering Article 50 -- the treaty mechanism used to exit the bloc -- two years prior.

May has warned that if MPs vote down her plan the country risks crashing out on this date with no agreement.

That would sever ties overnight with Britain's closest trading partner, amid fears of grounded flights, medicine shortages and gridlocked ports and motorways.

If May loses Wednesday's vote, most of the favourites to replace her are pro-Brexit, increasing the chances that Britain would leave without a deal.

- Second referendum -

Calls for a new referendum now attract cross-party support from dozens of MPs.

May has repeatedly ruled out another vote, but could face pressure to call one if Britain slips into political paralysis.

Supporters of a second referendum received a boost from the European Court of Justice on Monday, which ruled that Britain has the unilateral right to revoke its Brexit decision.

It could take weeks to elect a new leader if May is deposed, and Justice Secretary David Gauke warned on Wednesday that this could lead to the deadline being extended.

- General election -

The prime minister could try to break the parliamentary deadlock by calling a general election -- but would need the backing of two-thirds of all MPs.

Even if she survives Wednesday's party vote, a simple majority of all lawmakers could also topple her government with a vote of no confidence, with some opposition MPs on Monday calling for such a move in parliament.

But a Labour spokesperson said that the party would only submit such a motion "when we judge it most likely to be successful".

Losing a confidence vote tabled by the opposition could lead to the formation of a new government -- possibly a coalition of parties with a new leader -- if MPs agreed on it within two weeks.

Otherwise, a general election would be called.