Investment in Britain's key automotive sector collapsed by almost half in the first six months of the year owing to Brexit uncertainty, industry data showed on Tuesday.

The Society of Motor Manufacturers and Traders said car investment plunged to £347.3 million ($461 million, 395 million euros) in the first six months of 2018, compared with £647.4 million in the same part of 2017. The organisation warned that new investment projects were being hindered by uncertainty over the government's tough Brexit negotiations with Brussels.

"That's the cost of uncertainty, the price we pay for slow decision making," SMMT chief executive Mike Hawes told a press conference. "The (UK) government has set red lines on leaving the single market and the customs union. These red lines go directly against the interests of automotive."

Hawes added: "If we cannot cut through this current fog of uncertainty there will start to be real casualties."

The UK car industry employs 856,000 people and exported 1.33 million cars last year, according to the SMMT. Addressing Tuesday's conference, BMW's UK head Ian Robertson dismissed suggestions that the German car giant would move UK production abroad owing to Brexit uncertainty.

However he stressed that the group did not want to see delays to its European supply base, or car parts arriving into the UK from the continent.

"If we do, then we will clearly have to make investments to overcome some of it and that makes the industry less competitive," Robertson said.