The Helsinki-based Centre for Research on Energy and Clean Air (CREA) said the rebound was likely due to industrial activity as the country attempts to kickstart the economy.

"There are early warning signs that China's recovery from the Covid-19 crisis is reversing air quality gains," it said.

Despite experiencing a major drop in February, the level of pollutants were higher in the 30 days ending 8 May on last year, the report said.

Industrial activity appears to be driving the rise as areas with large numbers of factories showed the biggest increase, it said.

The group analysed data from 1,500 air quality monitoring stations in China.