David Jones admitted Wales did benefit from EU membership but may be even better off if the UK left the union.
The Clwyd West MP told BBC Radio Wales the Conservative Party was "probably overwhelmingly Eurosceptic".
He claimed more than 100 Tory MPs would be prepared to defy the wishes of the Prime Minister and back withdrawal.
David Cameron has promised an "in-out" referendum after he renegotiates the terms of the UK's membership of the EU.
Mr Jones called on the prime minister to focus on the rules surrounding benefits payments to migrants and measures to strengthen UK law.
"My assessment of the Conservative Party is that it's probably overwhelmingly Eurosceptic," he said.
"I think that if the Prime Minister does not achieve what we would like him to achieve, then we will certainly push for Britain to leave the EU."
Mr Jones, vice president of Conservatives for Britain, which wants to reform Britain's relationship with the EU, dismissed claims that Wales had more to lose from leaving the EU than other parts of the UK.
"This of course is frankly the great lie of those who want Britain to stay in the EU, come what may, put out that parts of the country such as Wales would be poorer," he said.
"My view is that the parts of the country such as Wales that do benefit from the EU as they do at the moment would actually be not only just as well off but could potentially be even better off as could the country as a whole."
Claims that Wales has more to lose than other parts of the UK by leaving the European Union are "a great lie", a former Tory Welsh secretary has said.