Here's an extract from a previous interview with Azevedo with the FT:
Britain joined the WTO under the auspices of the EU and its terms of membership have been shaped by two decades of negotiations led by Brussels. If Britain voted to leave the EU it would not be allowed to simply “cut and paste” those terms, Mr Azevêdo said.
Britain would have to strike a deal on everything from the thousands of tariff lines covering its entire trade portfolio to quotas on agricultural exports, subsidies to British farmers and the access to other markets that banks and other UK services companies now enjoy.
“Pretty much all of the UK’s trade [with the world] would somehow have to be negotiated,” he said.
The WTO had never gone through such discussions with an existing member, he said, and even the procedures for doing so remained unclear. But the likely complexity of such talks, Mr Azevêdo said, made them akin to the tortuous “accession” negotiations countries go through to join the WTO. Even a small economy such as Liberia, which last year became the WTO’s 162nd member, took years to agree the terms of membership.
WTO analysis had calculated the cost of the additional tariffs on goods imports to British consumers [before new preferential trade deals are negotiated] at £9bn, while British merchandise exports would be subject to a further £5.5bn in tariffs at their destination.
FT.com