The Government has issued a warning as a wave of high air pollution sweeps across London.

The Department of Environment, Farming and Environmental Affairs (Defra) has forecast high levels of pollution for Hertfordshire and north London, warning this could potentially aggravate some health conditions.

By Friday, high levels of air pollution are expected across the area.

The elderly, those with asthma and people with lung and heart problems should try to avoid strenuous exercise outdoors.

A statement from Defra said said: β€œIn terms of North London and Hertfordshire, we are expecting to see moderate to high levels of air pollution on Friday. These high levels will be cleared by Saturday morning, although there may still be one or two moderate measurements throughout Saturday. These will return to low by Sunday.”