Lord Howe of Aberavon, who served as chancellor and foreign secretary in Margaret Thatcher's government, said the Prime Minister had made his position more difficult by his "repeated concessions" to the eurosceptics. He said Mr Cameron now must give a lead on the issue.
Mr Cameron is committed to renegotiating the terms of Britain's membership and then putting the outcome to a vote in an in/out referendum after the next general election if the Tories win power. But while the PM has said he wants Britain to remain in the EU, Lord Howe - whose resignation over Europe in 1990 triggered Baroness Thatcher's downfall as prime minister - said that it was not enough.
In a statement released by the pro-EU British Influence campaign, Lord Howe said: "The more people understand and experience the reality of Britain's place in today's interdependent world - an interdependence which Conservative policies in the 1980s did so much to promote - the less likely they are to want this country to leave the European Union.
"Sadly, by repeated concessions to the eurosceptics, the Government made its own position on Europe, and in Europe, more difficult. Now the Prime Minister and his ministers need to make the case for membership strongly and clearly."
British Influence released a poll of "opinion formers" showing 69% believed Britain should stay in the EU, against 23% who wanted to leave. Of those who want to remain, 81% thought the Government was not doing enough to make the case.