THERESA May will finally present her Brexit objectives in a speech today at Lancaster House. It will be the most important event since the referendum itself; the most significant departure for the British economy in over 40 years.

As she waves goodbye on our behalf to the European Union we need to be absolutely clear about what we are leaving.

The Prime Minister has made clear that, despite appeals from Nicola Sturgeon, Britain will also be out of the European single market and the customs union under which all countries in the area have common external tariffs.

Read more: Theresa May makes clear UK's departure from EU will not be 'half in, half out'

The single market is a unique free trade area that encompasses not just physical trade, but also services (everything from finance to industrial design) and the free movement of capital and labour.

Free movement means that countries can’t discriminate against employees applying for any job anywhere in the EU. It also means that workers have equal rights in any country in Europe, including the right to benefits such as holiday pay.

But the Mrs May is adamant that Britain will no longer accept free movement or the rulings of the European Court of Justice, a body that has nothing to do with the Human Rights Act and is there to police the rules of the economic club.

For most of us, especially in Scotland, it is hard to believe that all these institutions will be in the Brexit dustbin. Where will this leave Britain?

Well, the stock answer from the Government is that this liberates the UK to strike trade deals with the rest of the world. Britain will stand tall, as it did in the 19th century, master of her own economic destiny.

The first trade deal will be with America, according to Donald Trump, closing “a quick deal and a great deal”.

Read more: Theresa May makes clear UK's departure from EU will not be 'half in, half out'

This is fine, except that a massive trade deal between the EU and the United States, the Transatlantic Trade and Investment Partnership (TTIP) has been grinding on unsuccessfully for the last five years.

Any quick deal with America is likely to involve very tough terms, notably on issues such as GM crops, environmental standards and access by US companies to NHS contracts.

It is folly to believe that Britain will be in a stronger negotiating position than the EU itself. A measure of the kind of hard ball that can be expected from the author of The Art of the Deal is that Mr Trump is promising to place 35 per cent tariffs on imported cars.

Of course, there will still be trade with Europe, no one doubts that. But, again, on what terms?

Brexiters believe the EU will cave in and give Britain free access to the single market without having to subscribe to the free movement of labour and the rules of the Court of Justice.

The theory is that Europe needs us more than we need Europe, and that it will inevitably cave in.

The EU, it is argued, has already blinked over financial services.

The EU’s chief negotiator, Michel Barnier, appeared to suggest to MEPs last week that the EU wanted a “special deal” guaranteeing access to the City of London. But this isn’t how the EU functions.

Mr Barnier is in no position to offer any special deals without the express authority of the 27 member states.

They have made clear that access to the single market has to be conditional on acceptance of both the Court and free movement.

Read more: Theresa May makes clear UK's departure from EU will not be 'half in, half out'

If Britain is allowed a special deal on these issues then the other member states will want the same. That would destroy the EU.

This is why Phillip Hammond is making clear that Britain stands ready to resort to less cooperative trade policies; what the Labour leader, Jeremy Corbyn, calls a “trade war”.

Britain might try to dump cheap exports in Europe, while erecting lots of artificial barriers to EU imports.

The UK Government might well slash corporation tax and try to set up as a kind of Hong Kong-style tax haven on the edge of the continent.

This may be a prospect that appeals to those with an irrational fear of Europe, but it would be a sad end to Britain as one of the foremost champions of free trade in history.

And it would be a slap in the face to working class Brexit voters.

They could find themselves competing, not with the privileged and relatively secure workers in protected countries in the EU, but with workers in developing countries like Brazil and Mexico.

Welcome to Brexit Britain, where the devil takes the hindmost.