UK car workers at Rolls-Royce and Mini have had an email from German parent firm BMW, highlighting what it sees as the risks of a vote to leave the EU.
While stressing the decision is ultimately a matter for the British public, BMW highlights the "significant benefit" the company derives from the free movement of people within the EU.
Without that, it says, the employment base of the company could be affected.
Meanwhile, 200 small companies' bosses have signed a letter backing an exit.
A referendum on whether the UK should stay in the EU will be held on 23 June.
BMW's email - produced in full below - says the movement of people between the UK and Europe "allows the rapid transfer of expert knowledge throughout the Rolls-Royce Motor Cars and BMW Group networks, building the skill level of our UK workforce".
It adds: "Free trade is important for international business.
"Tariff barriers would mean higher costs and higher prices and we cannot assume that the UK would be granted free trade with Europe from outside the EU."
The BMW group employs about 8,000 people directly in the UK, all of whom have received the firm's email. The company says the UK is its fourth-largest sales market.
"It is the only place in the world where all three of BMW Group's brands - BMW, Mini and Rolls-Royce Motor Cars - are represented by manufacturing operations," it adds.
BBC business editor Simon Jack said the email would re-pose a question that was raised in the Scottish referendum, as to whether it was appropriate for companies who do not have a vote, but do have a voice, to get involved in politics.
The body representing UK carmakers says it members want to stay in the EU.
The Society of Motor Manufacturers and Traders (SMMT), which represents more than 30 manufacturers building 70 models of vehicles in the UK, said its members had "clearly stated" in a survey that pulling out of the EU could jeopardise jobs, investment and growth.
Its boss, Mike Hawes, said access to European markets was the main factor, as the industry relied on EU markets for selling roughly half of its production.
Separately, in an open letter, the bosses of small companies urged Britain to leave the European Union to give them the "flexibility and adaptability... key to our long-term success".
The letter urges Britons not to listen to "a minority of managers from Britain's largest companies" who want Britons to stay in the EU.
Last week, the heads of 36 FTSE 100 companies said in a letter published in the Times, which was organised by 10 Downing Street, that Britons should vote to remain in the EU.
Leaving the European Union would threaten jobs and put the UK's economy at risk, the chiefs of companies including BT, Marks & Spencer and Vodafone said.
But Leave campaigners pointed out that two-thirds of FTSE 100 firms, including Tesco and Sainsbury, did not back the letter.
The British Chambers of Commerce (BCC) called the EU referendum "a choice between the devil and the deep blue sea".
John Longworth, the BCC's director general, will tell its annual conference in London that people face a choice between "staying in what is essentially an unreformed European Union", or leaving the European Union "with all the near-term uncertainty and disruption that this will cause".
Business Secretary Sajid Javid and Chancellor George Osborne are due to address the BCC, with Mr Osborne expected to say leaving the EU would expose companies to the "worst of all worlds".
However, writing in The Times, employment minister Priti Patel, a leading Eurosceptic, said smaller and more innovative businesses were being held back by the EU.
"Entrepreneurs are increasingly suffocated by EU rules and have little influence in shaping them," she said.
Claims Britain would be frozen out of a free trade deal with the rest of Europe if it left were "risible", she added.
On Wednesday, Lord Rose, the former Marks and Spencer boss who heads the pro-EU campaign group Britain Stronger In Europe, clashed with MPs over statistics during a Treasury Select Committee hearing.
The BMW letter to Rolls-Royce workers
As the debate around the referendum to decide the future of the UK's European Union membership increases, it is an appropriate time to outline the Rolls-Royce Motor Cars and BMW Group position.
The decision on whether to stay in the EU or not is for British voters to decide on in June. However, as a wholly-owned BMW Group company, it is important for all Rolls-Royce Motor Cars employees to understand the view of our parent company.
The BMW Group believes that the UK is better as a member of the EU than it would be outside it. You will see in the media this week an open letter supporting the campaign to stay in the EU, signed by around 200 business leaders, including Member of the AG Board, Ian Robertson.
Free trade is important for international business. Rolls-Royce Motor Cars exports motor cars throughout the EU and imports a significant number of parts through the region.
For BMW Group, more than half of Minis built and virtually all the engines and components made in the UK are exported to the EU, with over 150,000 new cars and many hundreds of thousands of parts imported from Europe each year.
Tariff barriers would mean higher costs and higher prices and we cannot assume that the UK would be granted free trade with Europe from outside the EU.
When it comes to regulation, whether the UK remains inside the EU or leaves it, with Europe as the UK's largest export market by far, we would have to abide by European rules and regulations in any case.
We believe it's much better to be sat at the table when regulations are set and have a hand in their creation, rather than simply having to accept them.
Finally, we get a significant benefit from the easy movement of our people between the UK and Europe. This allows the rapid transfer of expert knowledge throughout the Rolls-Royce Motor Cars and BMW Group networks, building the skill level of our UK workforce.
Our employment base could also be affected, with skilled men and women from most EU countries included in the 30 nationalities currently represented at the home of Rolls-Royce here at Goodwood.
The debate around this subject will undoubtedly continue to build as we near the UK referendum on 23 June 2016.