Smith's threat to resign came as Labour peer Lord Desai, an economic professor, accused Brown of looking "weak and indecisive" and suggested David Miliband as a better leader.

Desai said during an interview with the London Evening Standard: "Gordon Brown was put on Earth to remind people how good Tony Blair was." He described Brown's leadership style as "porridge or maybe haggis".

A mysterious bomb-making experiment that ended with the accidental death of a government scientist has remained an official secret for more than five years, leaving his family in the dark about what went wrong.

Terry Jupp, a scientist with the Ministry of Defence, was engulfed in flames during a joint Anglo-American counter-terrorism project intended to discover more about al-Qaida's bomb-making capacities.

There has been no inquest into his death, as the coroner has been waiting for the MoD to disclose information about the incident. An attempt to prosecute the scientist's manager for manslaughter ended when prosecutors said they were withdrawing the charge, but said the case was too "sensitive" to explain that decision in open court.

The Guardian has established that Jupp was a member of a small team of British and US scientists making bombs from ingredients of the sort that terrorists could obtain.