London's smog alert has been hitched up as a cloud of dust from the Sahara is set to hit Britain on Thursday (December 17).

The capital has been placed on a Level 6 warning compared to it's usual Level 2 rating by the Department for Food, Agriculture and Rural Affairs (Defra) .

Toxic fog and dust is threatening to engulf the region, although eastern areas are due to be worst hit.

Sahara dust blew across London in March 2014, leaving cars coated in a thin layer .

Health experts have issued a warning to those with asthma and other conditions to stay indoors and avoid exercise.

Director of Advice Andrew Proctor said: "People with asthma need to take extra care.

"We are facing a perfect storm of asthma triggers, with air pollution and weather changes increasing the risk of potentially fatal asthma attacks for millions of people with the condition.

"Two thirds of people with asthma find that air pollution makes their asthma worse and even more tell us that changes in the winter weather can trigger asthma attacks."

Temporary short term effects of being exposed to smog include discomfort and irritation to the nose, eyes, throat and can bring on headaches, nausea and dizziness.

The Saharan dust comes as London basks in what forecasters say could be the hottest December ever recorded.