Leaving the EU would be 'sheer folly' - John Major
The former prime minister said it would be "reckless" to consider leaving what he called "the world's pre-eminent trading bloc".
Writing in the Sunday Telegraph, he also argued that on issues such as the environment and consumer the UK was best served working "in unity with our fellow Europeans".
He also pointed to the security implications of leaving the EU, saying: "In an uncertain world the UK, as part of the EU, is better able to face up to the aggressive policies of hostile nations.
"We are safer, because the EU has brought together former enemies to face common perils. In the last thousand years of history, no previous generation has been so fortunate.
"It would be sheer folly to put this all at risk."
He also took aim at the claim from Leave campaigners that the UK would be able to negotiate a suitable trade deal having left the bloc.
Sir John argued it was the UK that needed the EU, as 45% of British exports go to member states, with only 7% of EU exports coming here.
"Their argument is that the EU needs the UK market more than we need theirs, on the basis that – overall – the EU exports more to the UK than we export to them. This is, at best, disingenuous. More bluntly, it is fantasy," he said.