LONDON -- British and U.S. intelligence agencies and police foiled a plot to create a chemical vapor bomb in Britain, the British Broadcasting Corp. said on Tuesday.

The alleged plot involved osmium tetroxide, a catalyst used in industry, but there was no indication that the suspected plotters had obtained any of the substance, the BBC said, citing security sources.

London's Metropolitan police said they "were not prepared to discuss" the report.

It wasn't clear whether the report was related to the arrest of nine British men last week and the seizure of a half-ton of ammonium nitrate fertilizer, which has been used in terrorist bombs in the past.

The plot apparently was to mix the chemical into a bomb, which would create a toxic cloud upon detonation.

The U.S. television network ABC reported Monday that U.S. and British intelligence had discovered the plot. ABC suggested that osmium tetroxide would cause victims to choke to death.

However, Alistair Hay, professor of environmental toxicology at Leeds University, said osmium tetroxide was "mildly irritant."

"It isn't like any traditional chemical warfare agent. It's not something which would create a major vapor hazard -- it isn't like mustard gas or chlorine or some of the nerve agents," Hay told the BBC.