Lord Bamford told employees he is confident the UK's manufacturing sector and JCB will continue to prosper whether the country is in or out of the EU.
He said: "In 1973, when we joined as its eighth member, the EU accounted for 31 per cent of world economic output. Today - with 28 member countries - the figure is just 17 per cent, which underlines the shrinking role of the EU in the world economy.
"So do I wish to remain in an EU of diminishing economic importance as it moves towards ever closer union? Or do I want us to pull out of the EU, reclaim our sovereignty and regain control of how we trade with Europe and the world? After more than 40 years in the EU, I will be voting to leave."
Meanwhile Green MP Jenny Jones has come out in favour of a Brexit, in contrast to most of her fellow Greens. Writing in the Guardian Jones said she believes the EU's "environmentally damaging" agricultural policies outweigh its positive impact on the environment.