Workers at Porsche’s factory in Stuttgart. About 42,500 jobs in ­Germany depend on supplier ties with Britain

A worst-case Brexit would threaten 14,000 jobs at German car parts suppliers and cut revenues by €3.8 billion a year, a study has warned.

The imposition of tariffs if Britain left both the single market and the customs union would lead sales in the EU and UK to fall by 770,000 cars, Deloitte said in its latest Brexit briefing paper.

European cars would become 21 per cent more expensive in the UK if the full costs incurred by a hard Brexit were passed on. German exports of completed cars and parts to the UK would drop by 23 per cent.

The UK is the largest export market in Europe for German car suppliers, with 18 per cent of parts used in British factories coming from