Environmental groups have renewed calls to reduce pollution on the 50th anniversary of London's Great Smog, which killed thousands of people.

The National Society for Clean Air (NSCA) said air pollution could be killing at least twice as many people as the official estimates state.

Meanwhile London Assembly members and some inner London boroughs want the mayor to make the capital a low emission zone.

The Great Smog of 1952 is estimated to have killed around 4,000 people - mainly through respiratory or cardiac problems.

Road, rail and air transport were brought almost to a standstill and a performance at the Sadler's Wells Theatre had to be suspended because fog in the auditorium made conditions impossible.

It was caused by fog combined with smoke from coal fires and industry.

The choking smog enveloped the city for four days - prompting the introduction of the Clean Air Acts of 1956 and 1968 which cut smoke emissions.

Emerging health evidence shows new sources of pollution provide a continuing threat

Richard Mills, NSCA

Nothing on the scale of the 1952 smog has happened again but experts say the present levels of air pollution, although not as visible, may be just as much of a threat.

NSCA secretary general Richard Mills said: "Emerging health evidence shows new sources of pollution provide a continuing threat and official figures may seriously under-estimate the level of death and illness."

The government estimates that 24,000 people a year have their lives shortened by air pollution but the NSCA believes it could be double that number.

It wants the government to reassess the health impact of air pollution.