Britain leaving the EU would actually diminish its sovereignty, ex-Taoiseach John Bruton has said.
The former EU ambassador to Washington said London would be stripped of its huge influence at the international negotiating table on key global issues if voters back a Brexit.
"Membership of the EU gives the UK a seat at the table when international trade agreements are being made," he said.
"The EU helps set the rules for global services and goods trade, and if the UK opts to leave the EU, its voice will simply not be heard when it comes to setting those rules.
"Opting out of the EU would thus diminish UK sovereignty."
Mr Bruton also claimed the loss of international influence could be "catastrophic" for both businesses and shoppers in the UK.
He was speaking at an investment seminar organised by Appian Asset Management in Dublin.
Mr Bruton said Britain also needed to heed its deficit - the deepest since 1955 - as it mulls leaving the EU ahead of the in/out referendum on June 23.
"The UK is spending more abroad than it is earning there," he said.
"It is making up the difference with borrowing.
"A country that is borrowing more abroad must be particularly sensitive to the volatile opinions of foreigners."
He predicted turnout would be crucial to the result.
"Young voters want to stay in the EU, but only 43% of under 25-year-olds voted in the last UK General Election," he told the seminar.
"In contrast, older people want to leave the EU, and in the last election, 75% of them voted."