The chief executive of energy giant EDF Energy has written to his UK staff outlining what he says are the benefits of continued EU membership.
Vincent de Rivaz says remaining would benefit the UK economy, but that Brexit could mean "Break It" for the EU.
EDF Energy employs more than 13,500 people across the UK.
Vote Leave said it was "entirely unsurprising" because EDF was owned by the French government, "whose policy is for the UK to remain in the EU".
The letter comes days before the referendum vote on 23 June.
'Benefits of collaboration'
EDF Energy is one of the UK's biggest energy firms, supplying about 5 million residential and business accounts with electricity or gas.
It produces about one-fifth of the nation's electricity, and is proposing to build the UK's new Hinkley Point C number power station.
Mr de Rivaz, who has lived in the UK for 14 years, is not a British citizen and therefore does not have a vote. He says the choice is for "voters to make" but that he himself would like to see a Remain outcome.
"Britain, the historical trading nation, has understood the benefits of collaboration and working together, of co-operation and partnership," he said.
"This country has embraced globalisation as an opportunity despite the risks. As a result Britain is a modern country with a strong economy, the fifth-largest in the world. One of the strongest economies in the largest single market in the world.
"Being part of that European single market, Britain is in a position to benefit from the best of the two worlds, leading from inside, not from behind."
He also said that China had chosen to invest in the proposed Hinkley Point C (HPC) power station because it was "a strategic partnership with the UK and with France".
Mr de Rivaz added: "The European dimensions of this project have been very important to China's decision to commit."
A final investment decision on the £18bn project will be made after the referendum, but Mr de Rivaz said: "The absolute need for HPC will remain regardless of the outcome of the vote, and politicians on both sides of the debate recognise this."
A spokesman for Vote Leave said state-owned EDF was simply echoing the policy of the French government - that Britain should remain in the UK.
He added: "EDF is also dependent on subsidies from the UK government, which has a pro-EU position."
As Thursday's Referendum approaches, there has been a series of businessmen lining up to urge people to vote one way or another.
Earlier this month Lord Bamford, chairman of JCB, one of Britain's most successful manufacturers, wrote to his company's 6,500 employees in the UK to explain why he favours a vote to leave the European Union.
Last weekend, entrepreneur Sir James Dyson told the Daily Telegraph Britain would gain more from leaving the EU than it would lose.
However, last week, BT bosses and union leaders came out in favour of the Remain camp.
Big business has generally been in favour of staying in the EU, although survey's suggest that small businesses are evenly split.
Last week, a poll by the Federation of Small Businesses suggested that 42% of members "could still be swayed on how to vote" with 52% "saying they didn't feel they had the information they needed".