At the heart of the economic debate about whether Britain will be better off outside the EU is our membership of the single market.
This morning Britain's car industry spelled out its anxiety that access will be lost.
As it stands a company can build a car in Britain and move it to any one of the 27 other countries inside the EU without paying any export or import taxes.
It can move employees across its staff between its European factories.
Car-makers worry these freedoms may be lost and that an industry which supports 800,000 jobs in Britain would suffer as a result.
"Nonsense", says the Leave campaign who maintain that Britain can quit the EU, retain the parts of the single market they want (free movement of goods and services) and stop EU migrants from coming to Britain (end the free movement of labour).
Germany's finance minister Wolfgang Schaeuble appears to have just said that will not be possible.
Have a listen to the exchange at the British Chambers of Commerce annual conference.
Schaeuble clearly states that ongoing access to the single market would only be permitted if Britain agreed "to pay" in the EU budget and if we "accept freedom of movement" of people as well as goods and services.
This looks like a very significant intervention, not least because immigration is also at the heart of the EU debate.
For the first time the EU's most powerful member state is revealing the trade terms that Britain would be expected to agree to in the event of Brexit.
Those who want us to leave the EU believe Britain could secure a better deal than either Norway or Switzerland. Schaeuble suggests Britain can't - the arrangement would be very similar.