European Commission officials have issued a bleak assessment of Iran’s nuclear potential, all but surrendering to the inevitability that Tehran will develop enough weapons-grade uranium to make a nuclear bomb.

The three-page policy paper, presented to EU foreign ministers at the weekend by Brussels diplomats and subsequently leaked, said the European Union’s twin-track policy — offering Tehran rewards for renouncing its secret weapons program but threatening sanctions if it did not — had failed.

“At some stage, we must expect that Iran will acquire the capacity to enrich uranium on the scale required for a weapons program,” it said. “Attempts to engage the Iranian administration in a negotiating process have not so far succeeded. The Iranians have pursued their program at their own pace, the limiting factor being technical difficulties rather than resolutions by the U.N. or the International Atomic Energy Agency.”

British officials yesterday rejected the grim conclusion of the document, which was presented by the commission’s senior foreign policy representative, Javier Solana. They said European diplomats had detected the first signs of a “new tone” taken by Iranian negotiators.

Whitehall officials and British diplomats in Tehran believe that the Iranian leadership was stung by a U.N. Security Council vote last month to impose sanctions on Iran for its illicit attempts to produce fissile material. “One of the things we have noticed is how hard the sanctions vote hit the Iranians,” one British official said.

Iran has faced repeated setbacks in its efforts to enrich uranium in covert research facilities, but it has shown the capacity to learn from power failures, design flaws, and a basic lack of equipment to steadily expand its capacity to produce weapons-grade material.

European officials have spearheaded attempts to coax Tehran into giving up its research in return for help building a civilian nuclear power program and a generous aid package.