Success in Brussels, now the campaign to sell her Brexit deal at home starts in earnest.

British Prime Minister Theresa May's presenting UK lawmakers with a single choice: Back her deal or reject it.

The no deal option, May says, would lead Britain back to "square one" with "more division and more uncertainty".

(SOUNDBITE) (ENGLISH) BRITISH PRIME MINISTER, THERESA MAY, SAYING: "Today marks the culmination of a long and difficult process of negotiation between the UK and the EU".

Lawmakers are likely to make their choice in mid-December with a parliamentary vote.

May needs 320 of them, not necessarily all from her own party, to agree with her.

Reuters Political Correspondent William James says the maths doesn't look good.

(SOUNDBITE) (ENGLISH) REUTERS POLITICAL CORRESPONDENT, WILLIAM JAMES, SAYING: "So we've got a bloc of around 80-90 perhaps euro skeptics.

You'll probably be able to whittle that number down somewhat but it's certainly going to be a big number.

On the other side it's it's much smaller it's maybe 15-20 people who who have said they're going to vote against it because they want to stay in the European Union.

But the prime minister has no outright majority in parliament.

The Northern Irish party, The DUP, usually prop up her government in votes.

They've said they're not going to back this deal either." If May's plan is voted down by parliament, the UK could then simply crash out of the EU with no deal.

Especially as fellow European leaders have already made clear they won't reopen negotiations after signing off on May's deal on Sunday (November 25).

But there are other likely scenarios.

(SOUNDBITE) (ENGLISH) REUTERS POLITICAL CORRESPONDENT, WILLIAM JAMES, SAYING: "The government has 21 days to come up with an alternative plan.

The reality is politics will overtake that.

We might see something very quickly afterwards.

The options are that May resigns.

We've seen that that's not in her nature she's she's very steadfast in kind of trying to see these things through.

They could try and come back for another vote." May's strategy has up until now been to push the pros of her deal.

Emphasising that it takes back control of money, laws, and borders.

Brexit campaigners though say it leaves Britain as little more than a vassal state: Unable to break free from Brussels, but still subject to its regulations.