British and US intelligence services foiled a plot to detonate a chemical bomb in Britain by a group thought to be sympathetic to al Qaeda, the BBC reported on Tuesday.
Security services intercepted communications between the potential attackers, who were planning an attack with a highly toxic chemical called osmium tetroxide, the BBC said on its website.
The potential target was thought to be areas in which there would be concentrations of people, possibly within a confined space, the BBC added, without naming a source for its report.
"The plotters were thought to be sympathetic to the aims of al Qaeda and the intended target was believed to be British civilians," it said.
Police in London however declined to comment, with a spokesman saying: "We're not prepared to discuss" the reports.
Osmium tetroxide is used mainly in research laboratories which can attack soft human tissue and could blind anyone who breathed in its fumes or cause them to choke to death in agony, British and US press reports said.
The US television network ABC, meanwhile, said those behind the plot were believed by British authorities to have been among terror suspects arrested in London last week. It said it was the first time osmium tetroxide has been linked to possible terror use.
"It's a nasty piece of work," Dave Siegrist, a bioterrorism expert at the Potomac Institute for Policy Studies in Arlington, Virginia, told the network. "It irritates the eyes, lungs, nose and throat. It leads to an asthma-like death, what we call a 'dry-land drowning.'"
US officials say the likelihood of a chemical bomb is much greater than a biological and radiological one. But the United States has still not settled how to tighten restrictions on what are known as toxic industrial chemicals, which are well-known to the al Qaeda network and still easily available, the ABC report added.
Nine people were arrested in and around London last week in operations which apparently foiled a plot to build and detonate a powerful bomb.