The European Union is, among many other things, a large free trade block with few barriers to cross-country trade within it. The United Kingdom has voted to leave the EU and is currently trying to negotiate the terms of that departure including the future terms of trade with the remaining block.
This isn't going well. The future options for trade range from staying in the "single market" (that is preserving the current trading terms as some other non-EU countries like Norway and Switzerland do) to just leaving the union with no deal at all and reverting to the default World Trading Organisation (WTO) terms of trade (see this Economist explanation of what this means).
One of the reasons for problems in the negotiations is that the government is split on which option is best. Some argue that no-deal (ie WTO terms) will be a disaster; others seem to think it is the best option.
So I was surprised to read the following aside in an article arguing that the market for financial services will not suffer much as the UK's position is so strong that the EU need the City of London more than the City needs the EU:
The worst-case scenario is no-deal. Whilst Britain certainly does not want that, we must remember that 90 per cent of world trade is conducted under WTO terms – the no-deal scenario.
While the article itself was making a case specifically about financial services, the claim is much broader. Clearly the claim matters as it bolsters the idea that a no-deal Brexit might not be that bad. Other sources are skeptical. For example, this analysis of the WTO database suggest that only one country (Mauritius) trades solely under WTO rules. The BBC summarises some of the conventional thinking here.
WTO rules don't sound so scary if everyone else is already trading under them. But the article offers no references for the 90% claim. So is it true? Is 90% of world trade under WTO terms?
PS The debate about whether Brexit is a good idea is controversial. Lets keep this question to the factual point at issue and not stray into the broader debate.