The UK government has urged medicine suppliers to build up stockpiles to prepare for possible border disruptions when Britain severs ties with the European Union at the end of the year.

In a letter published late Monday, the health ministry set out how firms could prepare for the end of the Brexit transition period on December 31 and the introduction of new customs controls.

"We recognise that global supply chains are under significant pressure, exacerbated by recent events with COVID-19," the ministry's chief commercial officer, Steve Oldfield, wrote.

"However, we encourage companies to make stockpiling a key part of contingency plans, and ask industry, where possible, to stockpile to a target level of six weeks' total stock on UK soil."

Britons voted to leave the EU in a June 2016 referendum and after years of political debate in London, the country formally ended its 47-year membership on January 31.

But it agreed with Brussels to a standstill transition phase until the end of 2020 to allow both sides to negotiate a new trading relationship.

Despite months of talks, the two sides remain at odds on some key issues, raising fears Britain will leave the EU's customs union and single market without anything to replace them.

The government stockpiled some key products including medical devices ahead of January 31, and Oldfield said it would build this store back up to a six-week level before December.

The letter also advises medical suppliers to plan on how they might re-route supplies in case of disruption at major ports such as Dover.