The first two phases of the scheme target the most polluting heavy vehicles and will be maintained by the mayor due to their effectiveness.

However, the third phase, which focuses light transport such as vans and minibuses, has been suspended due to the extra improvement costs, daily charges and fines it would impose on small enterprises.

Johnson comments: ‘Simply put, the cost of fitting pollution equipment or getting a new vehicle would have come as [a] punch in the ribs to those who need our help at this time, would have destroyed profit margins and endangered our businesses.’

The mayor claims that the Low Emission Zone has been successful in tackling the capital’s worst polluters, but said the timing is not right to expand it to smaller vehicles.

Introduced in February 2008, the scheme mainly affects older diesel-engine lorries, buses, coaches and other heavy vehicles and is designed to deter them from driving in the city.