The chief of the Vienna-based International Atomic Energy Agency, Mohamed ElBaradei, criticised Iran last night for persistently failing to disclose details of its nuclear projects.

In a 15-page report Dr ElBaradei said that, despite two-and-a-half years of intensive international investigation, questions remained about key aspects of Iran's clandestine nuclear activity.

The report was ordered by Britain, France and Germany, who have been seeking to mediate a settlement of the nuclear dispute with Iran for two years.

It has left the way clear for the three countries to declare an end to negotiations and join the US in trying to take the row from Vienna to the UN security council in New York, which has the power to impose sanctions on Iran.

Despite 30 months of "intensive inspections and investigations", said the 15-page report obtained by the Guardian, UN inspectors were still unable to "clarify some important outstanding issues".

"Iran's full transparency is indispensable and overdue," said the unusually critical report, supplying ammunition for the US-led drive to take the dispute to the security council.

For the past two years the Europeans have resisted US pressure, but they now appear to be running out of options.

Dr ElBaradei confirmed that activities at Iran's uranium conversion plant at Isfahan were proceeding. Almost seven tonnes of the processed uranium gas that can be enriched into nuclear fuel or weapons-grade material has been produced

The Iranians had frozen the Isfahan operation pending the outcome of negotiations, but restarted operations last month, triggering threats from the EU and causing them to order last night's report.

Last month the Iranians were asked to reinstate the Isfahan suspension otherwise the EU talks would be ditched.