[NeedMoreTea]

In comparison to being in the single market, yes it is. At moment of EU exit we lose ALL trade arrangements as all we currently have are under the auspices of the EU. That would, for a couple of years until treaties were getting signed, put us in a worse position than every WTO member and observer.

In the first instance NO countries trade solely on WTO default terms. None. Every country has better than WTO arrangements with someone, even North Korea who aren't even in the WTO. Perhaps their local neighbours and major trade partners, even if they don't necessarily have a deal with the EU (or China or USA).

On WTO terms we'd need to negotiate tariff agreements with the WTO on potentially hundreds of categories of products which is going to take at least as long as an EU-UK treaty. As frequently reported, goods and services to and from the EU will be subject to import duties. For any of the numerous industries where that may happen multiple times, e.g. car manufacture, the effect would be disastrous.

Second we'll lose all trade agreements with other nations that we have had as a result of being within the EU, e.g. with Canada, the US, South Korea, etc, etc. So we'll have no improved relationship with anyone at all. It remains to be seen if claims of interim arrangements are anything other than empty rhetoric, so we still potentially face that cliff edge in Dec 2020.

Third we'd lose the unrestricted single market right to provide services (including financial) across the EU without discrimination and fall back to only WTO terms. That's likely to completely change viability of numerous successful UK businesses. Including some of the largest.