China has detained five local officials after they jammed cotton gauze into sensors used to monitor pollution in an attempt to improve air quality readings, media reported.

The environmental officials had also tampered with computers to alter the results of pollution monitoring in the northern city of Xi’an, reports said.

China has vowed to wage a “war on pollution” and Beijing has set targets for local governments to cut levels of smog.

Authorities are also trying to collate emissions data to help them punish high-polluting businesses, but ensuring information is reliable has proved a challenge.

A member of staff from the Environmental Protection Bureau in Xi’an’s Chang'an district confirmed to The Telegraph that police had detained a group of officials in an investigation into falsified air quality data.

The Global Times newspaper citied local reports saying: “Officials in charge of environmental protection blocked the samplers to make the data ‘look better’ and avoid penalties for high pollution in their area of responsibility.”

Readings of PM2.5 – toxic particles small enough to deeply penetrate the lungs – can decrease by 30-50 per cent if cotton is used to cover air-monitoring equipment, an expert cited by The Beijing News said.