International inspectors have found that Iran has produced and experimented with polonium, a radioactive element that can help trigger a nuclear blast.

Western diplomatic sources told the BBC that while Iran still insists it had no clandestine weapons programme, the discovery does raise new questions. Iran was previously forced to concede it had not disclosed full details of its centrifuge technology. Centrifuges have a vital role in the uranium enrichment process.

Iran had agreed to fully disclose its past nuclear activity

Polonium-210 is a radioactive metallic substance that does indeed have a number of industrial uses. The discovery that Iran has both produced and experimented with the substance has nonetheless caught the attention of nuclear weapons experts. This is just the latest example of a nuclear activity which Iran has failed to declare to International Atomic Energy Agency inspectors. Polonium can be used in conjunction with another metal - beryllium - to ensure that the chain reaction leading to a nuclear explosion is initiated at the correct moment.

Worrying questions

In itself, this does not prove one way or another that Iran has or had a nuclear weapons programme. But it does raise some worrying questions in the minds of inspectors. And, according to western diplomats, it underlines the need for Iran to make a full disclosure of its past nuclear activities. The Iranian government agreed to do this late last year. But this most recent disclosure will have increased the pressure on the Iranian authorities to explain themselves more fully. Inspectors are not just seeking information from Iran. Libya has agreed to give up all of its nuclear weapons-related activities. In the process, western intelligence agencies and the IAEA have been able to lift the veil on the shadowy nuclear export operation run from Pakistan by the scientist Abdul Qadeer Khan. Some experts believe there could be links between his operation and Iran, in which case the Iranian authorities could have more embarrassing nuclear questions to answer.

Q&A: Iran's nuclear programme

There are fears Iran may develop nuclear technology for weapons The United Nations nuclear watchdog has condemned Iran for covering up nuclear activity and Iran says it will allow more intrusive inspections. News Online examines the issues involved. How had Iran broken the rules? The head of the International Atomic Energy Agency (IAEA) Mohamed ElBaradei said that Iran concealed activities which should have been declared under its IAEA agreement - namely "experiments in enriching and processing". This refers to the production of small amounts of enriched uranium and plutonium. However he said that Iran had now adopted a policy of full disclosure and that there was "no proof to date that Iran's past undeclared activities have been linked to a nuclear weapons programme". The Iranians said it was part of experiments in reprocessing chemistry. Enrichment is not banned under the Nuclear Non-Proliferation Treaty (NPT), but it must be notified.