In hard-hitting comments, which have sparked accusations of intimidation among eurosceptics, he said the UK would “have to bear the consequences” if it voted to quit the 28-member bloc.
In the event of Britain breaking away, it “would be considered a third party and one we won’t suck up to”, he added.
The tough-talking by the commission president is set to inflame the already rancorous referendum campaign being waged in the UK.
“If the British say ‘no’ and I hope they don’t, (European) community life won’t go on like before.
“The UK will have to accept it’ll be considered a third party and one we won’t suck up to.”
He added: “If the British leave Europe – both us and them – will have to bear the consequences.
“This isn’t a threat, but our relationship won’t be as it is today.”
He also said reforms secured earlier this year by David Cameron in a renegotiation of Britain’s membership of the EU would be implemented if the UK decided to stay in.