LONDON (Reuters) - With Britain sourcing the vast majority of the insulin needed by its 1 million diabetics from overseas, its biggest providers have had to restructure their supply chains in case a chaotic Brexit disrupts the normal arteries of trade.

Below are some of the steps Novo Nordisk, Eli Lilly and Sanofi have taken to prepare for Britain's departure from the European Union on Oct. 31.

- All three firms have increased the amount of insulin they are holding inside the country in case a no-deal Brexit leads to customs checks and lengthy delays at Britain's biggest ports of Dover and Folkestone.

- Denmark's Novo Nordisk says it has 18 weeks of supply which, if piled up, would stand at 12 times the height of the London Shard skyscraper. The other two have similar stockpiles.

- Both Novo Nordisk and France's Sanofi have reserved capacity on two additional ferry services to enable them to avoid snarl-ups at the main port of Dover if needed. They have also reserved air freight capacity.

- Sanofi has also divided its British and Irish supply chains to prevent any problems in southern England from hitting its Irish customers.

- Both Novo Nordisk and Sanofi have worked with their logistics providers to test everything from how long insulin should be held in temperature-controlled trucks, to what happens in the event of fuel shortages.

(Reporting by Kate Holton and Paul Sandle; Editing by Pravin Char)