Efforts to clean up London's chronic air pollution problem appear to be paying off, with City Hall policies helping spur a significant reduction in illegal nitrogen dioxide (NO2) levels over the past four years, according to the Mayor of London.

Latest air monitoring data for London released on Friday showed that last year only one site in London recorded a breach of both national and EU legal limits for NO2 on just over 100 occasions, marking a 97 per cent reduction on the 4,000 breaches recorded across the city in 2016.

Moreover, long-term exposure to air pollution appears to have declined, with monitoring sites in London overall seeing annual average NO2 levels drop by over a fifth between 2016 and 2019, the City Hall data reveals.

The Mayor's Office said even notorious hotspots for dirty air in the capital, such as Putney High Street in Wandsworth, Brixton Road in Lambeth, and Oxford Street in Westminster - all of which have high levels of road traffic, particularly from buses - remained within legal limits for NO2 on many more occasions last year compared to 2016.

Monitors on Oxford Street in particular - one of the capital's busiest shopping streets which is frequented by a long line of buses throughout the day - did not record a single breach above the hourly legal limit for NO2.

However, annual average pollution levels on Oxford Street and in other parts of the capital continue to exceed legal limits and pose a threat to human health, conceded City Hall, which said more needed to be done to clean up London's dirty air.

Nevertheless, the Mayor of London, Sadiq Khan, claimed the boost in air quality during his time at City Hall was a direct result of policies introduced since he was elected in 2016, including the expansion of the Ultra Low Emission Zone (ULEZ) for vehicles in the capital and the increasing rollout of electric and low emission buses.

"Toxic air is a national health crisis contributing to thousands of premature deaths ever year," he said, adding that the latest data showed it was "undeniable" such policies were making a positive difference to the air Londoners breathe.

Many parts of the UK continue to record levels of air pollution which exceed regulatory and recommended health limits, an issue which has seen the government repeatedly taken to court by environmental activists in a bid to drive more ambitious policies to clean up the air.

The government has launched a raft of clean air strategies in recent years, pledging to rollout more electric vehicles and provide more funding for electric buses. Last week it also unveiled plans to ban the sale of the most polluting fuels - such as coal and wet wood - for home fire places in a bid to tackle fine particle and dust pollution.

But while Khan said more needed to be done in London to cut pollution, he also called on the government to step up its own ambition to help London and other regional authorities tackle the problem.

"We're doing all we can in the capital, with proven results, so there are no excuses left for the government's failure to match our levels of ambition," he said.