The Builders Merchants Federation has written to Khan urging him to urgently reconsider the plan, which they say could turn Greater London into a “no-go area” for SME builders, and delay its rollout by a year.

The ULEZ is set to be expanded on 29 August, imposing a £12.50 daily charge on all vehicles which do not meet certain emissions standards, which includes all diesel vans registered before September 2016 and all petrol vans registered before January 2006.

A significant proportion of SME tradespeople working within the M25 use non-compliant vehicles, with many relying on diesel HGVs to move heavy materials such as bricks and timber.

The BMF’s letter said it was concerned that Khan’s officials have “no real understanding of commercial vehicle fleets” and had not allowed sufficient time for builders merchants and their trade customers to make operation changes.

The added charges are expected to result in many small builders choosing to buy materials outside the capital. This would put merchants within the new ULEZ boundary which serve housing projects in London at risk, the BMF said.

One member of the trade body said up to 40% of its staff would be hit by the charges, while another said up to a quarter of their daily business done through affected branches would be goods collected by customers who would have to pay the charge.

Another firm calculated that using a non-compliant vehicle five days a week would result in £3,000 of annual charges, representing an effective pay cut of around 14%.

John Newcomb, chief executive, BMF, said the burden would result in price hikes for end customers and affect staff levels and the amount of stock held at builders merchants within the ULEZ boundary.