Sir John told an audience of experts and journalists at the Chatham House think-tank, "Being inside the EU can often be frustrating; but outside, we would be at a serious competitive disadvantage."

　　"To leave, would be a jump into a void," he said.

　　He added, "The impact of withdrawal goes beyond trade and investment. In a world of global markets, countries are increasingly banding together."

　　"We are 70 million in a world of 7 billion, with nations like China, India, Turkey, Indonesia, Mexico and Russia all rising in economic and political importance. Do we wish to become less relevant in their eyes, or face the future as a leading voice in a block of 500 million people? To leave would be to march against history," he said.

　　He was speaking in the wake of the announcement last month by current Prime Minister David Cameron that he would seek to renegotiate Britain's EU membership, and put continued membership to a referendum by 2017.

　　Sir John was British prime minister from 1990 to 1997 and experienced dissent at the highest level within his Conservative party over his pro-EU policies.

　　He warned Cameron that negotiations with other EU states over a restructured British membership should aim to engage with and to understand the position of those states, otherwise they would fail.

　　"If we enter them with the aggressive attitude of 'give us our way or we quit,' we will fail," he said.

　　If the negotiations fail, the referendum could be lost and Britain could slip out of the EU in frustration and by default, he said.

　　Sir John cautioned that a principal handicap of withdrawal would be the severe blow to trade.

　　"As a non-member, we would have no national trade policy in place, and would need to negotiate our own free trade agreements. At worst, this could take years. At best, we might be able to piggy-back on existing EU agreements. Either way, we will face tariffs," he said.

　　He cited car making in Britain as an example, which is dominated by non-British firms such as Toyota, Ford, Nissan, and BMW.

　　With 80 percent of British-built cars going for export, a 10-percent tariff on exports to the EU would be a hard blow for manufacturers to bear and could encourage them to relocate to continental Europe.

　　Sir John also warned that if Britain left, Europe would lose one of its biggest economies, and Europe's "role as a global power would be diminished."