Car manufacturers have warned that time is "running out" for the UK to agree a Brexit deal with the EU that will allow the British automotive industry to survive.
They called for swifter progress on a deal "that will put an end to uncertainty and prioritise the needs of the automotive sector".
The warning came from the Society of Motor Manufacturers and Traders, which represents carmakers in the UK. The industry directly employs 186,000 people while the wider industry, including component manufacturers and others, employs a total of 856,000.
Mike Hawes, SMMT Chief Executive, said the UK's current arrangements with the EU were the right ones for the car industry - and said there was concern that the Government still didn't know what would replace them once Brexit takes place.
He warned: "There is growing frustration in global boardrooms at the slow pace of negotiations".
It comes after carmaker BMW warned it could close plants in the UK, including its engine manufacturing facility at Hams Hill, Coleshill, near Birmingham.
The sites could go if BMW cannot quickly and reliably import components from mainland Europe after Brexit, the firm said.
At the moment, the UK's membership of the EU's Customs Union and Single Market allows parts to be imported and exported between the UK and the rest of the EU without delays at borders.
Stephan Freismuth, BMW's Customs Manager, told the Financial Times newspaper: "We always said we can do our best and prepare everything, but if at the end of the day the supply chain will have a stop at the border, then we cannot produce our products in the UK."
BMW employs more than 7,000 people at four facilities across the UK.
Carmakers have repeatedly raised concerns about arrangements at borders after Brexit, which is due to take place on March 29, 2019.
They tend to have complex supply chains involving a large number of firms which provide components, and many of these suppliers are in other parts of Europe.
They also operate what is known as a "just in time" system of manufacturing, in which parts are ordered when they are needed and arrive swiftly at the factory.
The system works because parts can be sent to the UK without delays, thanks to EU membership. But it would fall apart if there were long delays at borders.
Mr Hawes said: "The current position, with conflicting messages and red lines goes directly against the interests of the UK automotive sector which has thrived on single market and customs union membership.
"There is no credible ‘plan B’ for frictionless customs arrangements, nor is it realistic to expect that new trade deals can be agreed with the rest of the world that will replicate the immense value of trade with the EU. Government must rethink its position on the customs union."
Mr Hawes continued: "There is no Brexit dividend for our industry, particularly in what is an increasingly hostile and protectionist global trading environment.
"Our message to government is that until it can demonstrate exactly how a new model for customs and trade with the EU can replicate the benefits we currently enjoy, don’t change it."