"Our opponents believe because the government's majority is so small, that our resolve will weaken and that they can force us to change course," she said.

She challenged the opposition parties who have criticised the government's vision for Brexit to "put forward our plans for Brexit ??? and then let the people decide".

So what is her government's vision, which has been dubbed 'hard Brexit', that it feels it needs a bigger Westminster majority to defend? And what is being opposed?

Immigration

In her key Brexit speech in January, May said she intended to "get control of the number of people coming to Britain from the EU".

Record levels of net migration in the last decade had put pressure on schools and housing, and suppressed wages, she said.

"You cannot control immigration overall when there is free movement to Britain from Europe," she said. "The message from the public before and during the referendum campaign was clear: Brexit must mean control of the number of people who come to Britain from Europe."

One of the four pillars of the EU is the principle of free movement of labour. Britain will no longer agree to this.

But millions of EU citizens work in the UK, many in key sectors such as healthcare.

The government is considering a "barista visa", similar to the youth mobility visa currently available to Australians in Britain, under which young EU citizens could come for a few years and work. There has also been talk of a selective skills test for work visas.

Beyond this, the government has been vague on how it will control its borders. And there are special problems between Ireland and Northern Ireland, where peace might be threatened by the return of border posts.

Though the rhetoric is about entry into Britain, it may instead just come down to who can legally work there - a bureaucratic exercise imposing new red tape on businesses.

Labour's position on free movement into the UK for EU citizens is clear as mud.

In January leader Jeremy Corbyn said "Labour is not wedded to freedom of movement for EU citizens as a point of principle, but I don't want that to be misinterpreted, nor do we rule it out".

Many in Labour are opposed to an immigration policy that would sacrifice industry and the public sector for the sake of border control.

Labour's official position is that it wants "fair management of migration in the interest of the economy and communities", rather than free movement per se.

EU Trade

Before May's speech, there was wide speculation that Britain might stay a member - or at least an associate member - of the European single market, despite leaving the EU.

She knocked this on the head. "What I am proposing cannot mean membership of the Single Market," she said.

"European leaders have said many times that membership means accepting the 'four freedoms' of goods, capital, services and people. And being out of the EU but a member of the Single Market would mean complying with the EU's rules and regulations that implement those freedoms, without having a vote on what those rules and regulations are.

"It would to all intents and purposes mean not leaving the EU at all."

Instead May wants a free trade deal with the EU - and if one cannot be struck before Brexit with the EU (which is notoriously slow to agree such deals), to fall back to WTO terms.

It's a gamble. May herself, in a speech before the referendum, pointed out that WTO rules would oblige the EU to charge tariffs on UK exports - including 10 per cent on car exports. This would hurt British industry.

And financial services may flee to the continent once they lose 'passporting' that allows them to work seamlessly across European borders.

May wants a trade agreement to mirror the single market in some industries - she has cited automotive manufacture and financial services.

Many Labour MPs oppose a hard Brexit from the single market, arguing it would lead to fewer jobs, less business investment, and hurt consumers through higher prices.

Corbyn says "unfettered access" to the markets of EU member states was "fundamental" for Labour.

Global trade

In her January speech, May sought to fudge the question on whether the UK would remain within the EU's Customs Union.

Her government has repeatedly talked up the proposition of new trade agreements with countries outside Europe - including Australia. Full Customs Union membership would preclude such comprehensive trade deals, May said.

So she does not want a common commercial policy, nor a common external tariff system with the EU. She wants Britain to be free to establish its own WTO tariff schedules.

However she does want, she says, "tariff-free trade with Europe and cross-border trade there to be as frictionless as possible".

It's unclear how she can square this circle. She must conjure a new customs agreement, or remain a signatory to some elements of the Customs Union, but not others.

"I have an open mind on how we do it," she said, optimistically.

The reason she's keen to find a compromise is that industry groups are already howling about the damaging effect of reintroducing customs checks, which would be a likely consequence of leaving the Customs Union. They warn of slow queues of trucks at ports and reduced volume of trade, particularly affecting goods such as fresh food.

The government's own statistics forecast a resulting 4.5 per cent hit to UK growth.

Labour MPs have argued the UK should remain in the customs union, as the benefits of new trade deals with other countries could not possibly make up for the extra cost of trade with its closest neighbours in Europe.

Labour's official position, as set out by its leader, is that Brexit must deliver the "exact same benefits" as membership of the Single Market and Customs Union.

Implementation

At the end of March the government set out its plan for a "Great Repeal Bill" - which doesn't actually jettison all the hated EU legislation that Britain voted to reject.

In fact it does the opposite. The government will ask parliament to import, wholesale, 40,000 legal acts, 15,000 court verdicts and 62,000 international standards: all the EU law that currently applies to the UK.

But the government will give itself a 'Henry VIII clause', allowing ministers and civil servants to tinker with the fine print of that law without referring back to parliament.

May argued it is necessary to fix the huge amounts of law that will not quite work properly after it is naturalised.

But Labour is furious at the idea, saying it gives the government "essentially dictatorial powers" to set new Conservative laws, with the stroke of a pen, in areas such as the environment, health, consumer protection and employment rights.

The Great Repeal Bill could face a rocky road through parliament - unless the Conservatives have a more comfortable majority than the one they currently enjoy.