LONDON (BLOOMBERG) - The UK and European Union negotiators have reached a Brexit deal. But it is far from over yet. Here are the four things to know as it heads into the most perilous part.

WHAT'S THE DEAL?

It will be the most important international agreement in Britain's postwar history. It will set out the terms of separation that allow the UK to depart the European Union on March 29, 2019 in an orderly fashion - and bring with it a grace period to give everyone time to adjust.

It will probably also come with a political declaration that sets out how the two sides want to trade in the future - but the details of a free-trade agreement could take years to work out.

Prime Minister Theresa May has to get her Cabinet to sign off on it at an emergency meeting called for Wednesday at 2pm local time. But she cannot take their backing for granted.

SO IT'S TOO SOON TO CELEBRATE THEN?

Yes and no. For months, negotiations have been at impasse over what has become known as the Irish backstop. Pro-Brexit ministers will be worried whether it traps the UK in the customs union indefinitely.

So getting the Cabinet on board is one hurdle. Assuming Mrs May jumps it, a special EU summit could come end of the month - Nov 25 is a touted date - to get all leaders to give it their rubber stamp. The problems, as always, are back home.

Mrs May would still need to get it past Parliament, where the government does not have a majority and faces opposition on all sides. This historic decision is going to hang on just a handful of votes.

The biggest risk comes from about 50 furious pro-Brexit Tory lawmakers who think she has caved to the EU and betrayed the electorate's call to regain sovereignty.

There is also the danger that the 10 Northern Irish lawmakers propping up her government could shoot the deal down. Some pro-EU Tories are not very pleased either.

With the numbers this tight, Mrs May's whips are fishing for support in the opposition Labour Party while turning the screws on their own.

WOULD LOSING BE THAT BAD?

It depends. If Parliament rejects it, Labour will push for a general election. But the chaos that ensues would probably also provide the best chance for lawmakers to push for a second referendum.

A group of Members of Parliament from across the House of Commons are trying to engineer a re-run but so far there is nowhere near a majority for it.

There is a real chance that if the deal is voted down, the country will crash out of the bloc into a legal limbo that would snarl trade and freeze up markets.

Free trade between Britain and the EU will give way to basic World Trade Organisation tariffs and become subject to border checks.

Delays would be so bad that the government has plans to turn a major highway near the Port of Dover into a holding zone for trucks. Bottlenecks could bring shortages of everything from food to drugs and manufacturing components.

Financial contracts would also risk being void and there is a question mark over how derivative trades would be cleared.

HOW DOES THE CITY EMERGE FROM THIS?

The deal - once it is through Parliament - brings some relief to London's financial hub because the grace period buys them time. But because of the way the talks were structured, there is still not much clarity about how banks will operate in the future.

What is known is that there is a big downgrade from the status quo. That has to be expected as the UK had long dropped demands for close integration.

UK banks will have to use the same or similar regime to the one that US and Japanese banks use now - it is called equivalence and it has lots of pitfalls.

The full extent of the changes will emerge in the final trade agreement, which could be years from now.