Speth said a "bad Brexit" deal that reimposes barriers between Britain and its biggest trading partners "would cost JaguarLand Rover more than £1.2 billion ($1.6 billion, 1.3 billion euros) profit each year". "This would be in jeopardy should we be faced with the wrong outcome".
Mr Western, who has been having meetings with JLR bossess in recent weeks, said: "JLR are exasperated by the situation".
Government business minister Greg Clark said: "JLR is a great British success story".
"This is hugely important and it just shows how urgent this situation is".
Bernd Atenstaedt, the chairman and chief executive of GIUK, said it was frustrating for his member businesses because they still did not know what a post-Brexit Britain would look like two years after the referendum.
'For more than 250 years, since the era of Adam Smith, Britain has championed free markets and made the case for free trade.
"If we do not have a customs union, there are sectors of manufacturing society in the United Kingdom which risk becoming extinct", said Paul Drechsler, president of the Confederation of British Industry.
"Jaguar Land Rover and Tata Motors have always maintained that the uncertainties from Brexit are avoidable and the business seeks clarity to ensure that industry takes timely and right decisions to manage the transition", P.B. Balaji, Chief Financial Officer, Tata Motors Group was quoted as saying in a statement.
The company has already said it will move production of its Land Rover Discovery to Slovakia from Solihull by early next year.
New Land Rover cars are seen in a parking lot at the Jaguar Land Rover plant at Halewood in Liverpool, northern England.
It employs 40,000 people directly and 260,000 work in its supply chain. BMW also recently issued a similarly stark warning back in June.
A meeting on Friday of British government ministers to try to agree a position for negotiations with the European Union would take the concerns of business into consideration, he said.
JLR has joined the fray, saying the group needed "free and frictionless trade with the European Union and unrestricted access to the single market". The business secretary said the government would do all they can to keep Britain attractive for businesses. but Boris Johnson is also rumoured to have said "f*** business" at a dinner last week, so.