Being part of the EU means that British businesses has as its home market not just the UK, but the whole of Europe. The EU is home to 500million consumers who purchase nearly half of our exports - worth more than £200billion last year. The single market allows our businesses to sell to Europe without facing barriers in the shape of tariffs, red tape and differing regulations.
The jobs, livelihoods and opportunities of millions of Britons depend on this trade. Leaving would put them at risk. Whatever the Vote Leave campaign might say, there is no alternative that could even come close to replicating the economic benefits of EU membership.
Vote Leave have admitted today that they want Britain to leave the Single Market. They do not set out their alternative in detail - today, Michael Gove suggested, in vague language, that we would join "a free trade zone stretching from Iceland to Turkey".
This might sound nice. But the EU's single market is not a vague concept. It is defined in the treaties that govern the European Union. You are either in it or not. If we leave the single market, our country will face new tariffs on goods trade, which means higher prices, exclusion from the single market in services, which comprise 80% of our economy and would put jobs at risk, and the UK having no influence over EU regulations we would be forced to follow when trading.
Vote Leave have already come up with a panoply of countries they think Britain should emulate - Norway, Switzerland, Canada, even Peru. Today, they added a few more to the list - namely Bosnia, Serbia and Albania. These countries, which have bilateral free trade agreements with the EU, have a far inferior deal to Britain. Some tariffs on imported goods remain, and critically service markets remain outside their scope. Our vital financial services sector would lose the so-called "passport" that allows our firms to operate freely in Europe. Leaving the EU on these terms would see a flight of jobs from London, Edinburgh and Leeds to Dublin, Frankfurt and Paris.
Detailed and sober analysis released by the Treasury suggests that, under this scenario, the economy would be 6.2% smaller, costing the average family £4,300.
But things could be far worse than this if we leave, as Vote Leave today made clear. If Michael Gove is to be believed, we would repeal the European Communities Act 1972, rather than invoking the lawful Article 50 process. Article 50 at least mandates two years of negotiations on a new arrangement before the country involved actually leaves. But repealing the Act without first invoking Article 50 would see Britain dumped, unceremoniously and unilaterally, out of the EU. It would also be an unprecedented and flagrant breach of an international treaty obligation which as Lord Chancellor I would not have expected him to countenance.
Suddenly, we would have no free trade deal with the EU at all. This means we would only be able to trade with the EU under World Trade Organisation (WTO) rules. Tariffs on our exports would spring up, hitting exporting business and the jobs that depend on them. Tariffs on imports would also be likely to rise, causing higher prices in the shops. The Treasury estimates that this nightmare scenario would make our economy 9.5% smaller, costing the average family £5,200.
Aside from the dangerous weakness of its central argument, the speech contained numerous misrepresentations. EU countries do not have a huge incentive to conclude a generous free trade agreement with Britain, since just 8% of their exports are sold to us, while nearly half our exports are sold to them. And a vote to remain does not mean a vote for "more Europe", since the Prime Minister's renegotiation has delivered us solid and substantial safeguards against being caught up in further integration.
Britain is stronger, safer and better off as a member of a reformed European Union. Leaving would be a leap in the dark with a very high risk of seeing jobs lost, prices rise, and businesses shut out. Michael Gove had the chance today to try to set out a detailed plan for how he believes Britain could thrive outside Europe. Instead, his speech serves only to prove that no such plan exists. It is now clearer still that the sensible choice is to vote to remain in Europe as the key to our future prosperity, security and wellbeing.
Dominic Grieve is Conservative MP for Beaconsfield and a political champion of Britain Stronger In Europe. He served as Attorney-General from 2010 to 2014