The French company Sanofi has become the latest drugs manufacturer to start stockpiling medicines amid mounting uncertainty over the UK’s exit from the EU.

Supplies of thousands of medicines could be disrupted if Britain leaves the EU without a deal next March and border checks and lengthy transport delays arise.

The rival pharmaceutical firms AstraZeneca and MSD (known as Merck & Co in the US) have also announced stockpiling plans.

Sanofi, a large producer of insulin and vaccines, is building up stocks of a wide range of medicines in British warehouses, creating a 14-week supply starting from April 2019. Most of its supplies arrive in the UK through the Channel tunnel; it normally held just 10-week supplies of drugs in case of disruption, it said.

Hugo Fry, the managing director of Sanofi’s UK operation, said: “The uncertainty in the Brexit negotiations means that Sanofi has been planning for a ‘no deal’ scenario. Patient safety is our main priority and we have made arrangements for additional warehouse capacity in order to stockpile our products, where global supply allows, in the UK, and increase UK-based resources to prepare for any changes to customs or regulatory processes.”

He said this was in line with recommendations by the European Federation of Pharmaceutical Industries and Associations (EFPIA). “In the absence of any transition agreement, the European Medicines Agency will deem the UK to be a ‘third country’.”

The threat of a no-deal with the EU over Brexit is also forcing drug makers to prepare for duplicate product testing and authorisation to ensure their products stay on the market.

According to the EFPIA, about 45m patient-packs go to the EU from the UK every month and 37m patient-packs travel in the other direction.

The Anglo-Swedish drugs maker AstraZeneca has been boosting stockpiles of medicines by 20% in warehouses in the UK and at ports in the EU. Its chief executive, Pascal Soriot, said last week that the company was raising inventories from three months’ supply to four. He expressed concerns, saying: “We don’t even know how medicines will cross borders. Sometimes they cross several times during the manufacturing cycle.”

AstraZeneca, which makes the cholesterol pill Crestor, as well as the cutting-edge cancer treatments Tagrisso, Imfinzi and Lynparza, has begun parallel testing of drugs at its Swedish site in Gothenburg.

The American pharmaceutical group MSD also has plans to build up medicine supplies, with stocks lasting up to six months.

Last week, the health secretary, Matt Hancock, said NHS England was preparing to stockpile medicines and blood in case the UK left the EU without a deal. He told the health select committee he had met medical industry leaders to accelerate preparations.

Sir Mike Rawlins, chair of the Medicines and Healthcare Products Regulatory Agency, has warned that all insulin is imported to the UK and has to be temperature controlled.

GlaxoSmithKline, Britain’s biggest pharmaceutical company, which makes the asthma drug Advair, as well as HIV treatments and vaccines, said it always held additional supplies of medically critical drugs in case of disruption.

GSK started implementing its contingency plans in January, focusing on supply chains. The drug maker estimated that Brexit-related costs would be up to £70m over the next two to three years, with ongoing costs of £50m a year, due to re-testing of medicines, transferring marketing authorisations in the UK to the EU, and altered manufacturing licences and other steps.

A GSK spokesperson said: “Making sure that patients can access the medicines and vaccines they need is a priority for GSK, which is why we are continuing to implement our contingency plans to minimise the potential for disruption to supply of these products as a result of Brexit.”