Air pollution in London passed levels in Beijing this week, figures have shown, with popular wood burning stoves blamed for exacerbating the problem.

On Monday London mayor Sadiq Khan issued the highest air pollution alert in London for the first time, and said on Tuesday that the capital's 'filthy air' is now a 'health crisis.'

Readings at 3pm on Monday showed that air at locations in the capital were worse than in notoriously smoggy Beijing, hitting a peak 197 micrograms per cubic metre for particulate matter on the Air Quality Index. Pollution in the Chinese city only reached 190, which is still deemed 'unhealthy.'

Although nitrogen dioxide levels in London rose higher than China in 2014, it is believed to be the first time particulate readings have exceeded those in the far east.

Experts at King's College London said the recent spell of unhealthy pollution was the worst since April 2011 in the capital and was being caused by cold, calm and settled conditions combined with 'traffic pollution and air pollution from wood burning.'

Temperatures have fallen below zero overnight over the last few days, meaning householders are burning more fuel to keep warm.

"This was the largest contribution from wood burning measured during the winter so far," said a spokesman for King's College.

More than a million homes in Britain now have a wood burning stove with 175,000 new ones installed every year.