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 BBC said 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.
The U.S. Occupational Safety and Health Administration lists osmium tetroxide as causing difficulty breathing.
However, Alistair Hay, professor of environmental toxicology at Leeds University, told the BBC that 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.
Some 700 officers took part in the raids last week that nabbed the nine men and the half-ton of ammonium nitrate.
The men, aged 17 to 32, were arrested on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism.
Two suspects were arrested in Uxbridge, also in west London, and three in Crawley, south of the capital. One was detained in Ilford, east London, another in Slough, west of London, and another in Horley, south of the capital.
Uxbridge and Slough are near Heathrow airport, while Crawley and Horley are near Gatwick airport.
Officers conducted a total 24 searches that also targeted addresses in Reading, Luton and north London. There is an airport in Luton.
Ammonium nitrate is a common fertilizer, but it can be mixed with fuel oil to make a powerful explosive. It was used in the Oct. 12, 2002 blast in Bali that killed 192 people, mostly Western tourists.
The substance was also used to make a bomb in a van which was parked near the U.S. Consulate in Karachi, Pakistan, on March 15. It was defused before it could explode. It was the ingredient in a suicide bomb detonated outside a British bank in Istanbul, Turkey in November.
In years past, the Irish Republican Army used ammonium nitrate in attacks in London and in Northern Ireland. Ammonium nitrate was also used in the bombing of a government building in Oklahoma City which killed more than 160 people on April 19, 1995, and in the 1993 bombing of the World Trade Center.
Because of the decades long Irish insurgency, London authorities have far more experience dealing with terrorism than most of their American counterparts. In downtown London, for example, there are very few wastebaskets because of the threat of IRA bombs.
According to the Guardian newspaper, last week's arrests were part of a crackdown launched after Sept. 11 in which hundreds of suspects have been detained. Muslim groups have complained that only 77 people have been charged out of 500 who were picked up.