Drivers who leave their engines idling in central London could face a fine of £20 under plans to improve air quality.

Traffic marshals from Westminster City Council will patrol the streets of the borough, approaching motorists who sit in stationary cars with the engine running.

The Fixed Penalty Notice, which will go up to £40 if not paid within 28 days, will be a “last resort” if drivers refuse to turn off their vehicles on request from the traffic marshals. The measure will be introduced on 1st May.

London Mayor Boris Johnson introduced a number of no-idling zones in 2011, with traffic marshals targeting “hot spots” including taxi ranks outside major stations and coaches on Park Lane.

The new policy follows Camden Council’s fines for buses which idle for too long, introduced in 2011, and Islington Council’s fines for idling vehicles introduced in August last year.