HONDA bosses are warning of ‘unprecedented’ disruption to its supply chain if Britain leaves the customs union after Brexit.

The car manufacturer, which employs 4,000 staff at its Honda Civic plant in South Marston, says that leaving the customs union will lead to severe disruption which could force it to use sea routes, adding up to nine days to production times.

The company is heavily reliant on importing components for the plant from the EU via the Channel Tunnel, with 75 per cent of the two million components delivered to the plant every day arriving between five and 24 hours.

In the event of a Britain leaving the customs union, Honda expects it would need to handle 60,000 additional customs declarations, requiring a new IT system and additional staff. HM Revenue & Customs said this would cost Honda £2.1m a year alone in extra paperwork, even before the costs of a more unpredictable supply chain add up.

Speaking with the Financial Times, Ian Howells, senior vice-president of Honda Europe, said: “The breadth of what we are dealing with is unprecedented in terms of its total impact.

“With foreign currency you can react, you can see what the problem is and react to it. With Brexit we just can’t react.

“We can scenario, we can contingency plan, but we can’t then react because we don’t know.

“Shipping takes a long time, [but] that’s what we’re facing.”

Such a delay would present a logistical headache for the firm. At the moment the warehouse can only physically store enough parts to keep the production line in Swindon going for 36 hours.

To cope with such delays it would need to build a 300,000 square metre warehouse – the size of 42 football pitches – to keep enough parts in stock to last for nine days.

Ian Larrard, director of the Swindon and Wiltshire business initiative Business West, said: “Honda are saying clearly that Brexit will have negative consequences for the industry and may threaten their future UK production.

“Sadly this could threaten not just Honda’s jobs and investment, but thousands of other jobs in their supply chain. Without clarity now, businesses are being forced to plan for the worst-case scenario.”

Car manufacturers across the UK, including the BMW group, which employs around 850 people at the Mini plant in Stratton , have also warned of disruption and increased costs to production if new customs arrangements are introduced.

Exports to the EU account for more than half of all UK car exports.

Honda currently exports more than half its production line from Swindon to the US.