The luxury car firm, which has witnessed a major turnaround in its fortunes under the Tata Group's charge over the last few years, is believed to have conducted an internal analysis of the likely impact of Brexit, The Telegraph said.
The analysis expects the 1 billion pounds hit to come from a 10 per cent levy on vehicles being exported to Europe and 4 per cent on imports of components for the production of vehicles, the newspaper said.
However, the company's stance on referendum has been clearly in favour of remain in the run up to tomorrow's in-out referendum.
In a letter to JLR staff, company CEO Ralf Speth wrote that the automotive major was "convinced that we will be a stronger business - and a stronger country - if the UK remains a member of the European Union".
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"If the UK was to leave the EU, it would become more difficult to buy components and sell our products in our largest market," he said.
"Our European supply chain has been fundamental in helping us to meet customer expectations worldwide and achieve sustainable, profitable growth."
JLR's most recent annual results showed the company made 1.56 billion pounds pre-tax profit on revenues of 22.2 billion pounds.
Europe represented24 per cent of the total sales of 521,571 vehicles, making it the single biggest market for the company, ahead of the UK at 20 per cent.