A vehicle being produced at Nissan's factory in Sunderland in the U.K.

Automakers have called on the U.K. government to back down from its “red lines” in Brexit negotiations, or risk putting hundreds of thousands of jobs at risk.

In a statement released Tuesday, the Society of Motor Manufacturers and Traders (SMMT) CEO Michael Hawes said Downing Street should “rethink its position on the customs union” and work towards a Brexit deal that delivers “single market benefits.”

A customs union is an agreement that allows partaking countries to set common external tariffs, allowing goods to travel freely between those nations. The customs union which covers Europe was created in 1958 as part of the European Economic Community (EEC), which then evolved into the European Union (EU).

The single market is a deeper form of co-operation between member states that allows the free movement of goods, services, money and people within the bloc.

Hawes said Brexit uncertainty would provide “no dividend for our industry” and the current position with its “conflicting messages and red lines” was against the interests of the U.K. auto sector.

“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,” he said.