More than 40 businesses have signed an open letter to ask the mayor of London to extend the congestion charge exemption for electric vans.

Their special status means they currently pay nothing – but from Christmas the charge would be £15 a day.

Signatories, including Ocado, the AA and the Federation of Small Businesses, argue the charge – which would add up to £5,500 per vehicle per year – would undermine firms who had “taken on debt to invest in the air we breathe”.

City Hall said it was working with Transport for London to “see what more could be done to mitigate the effect of this phasing out and further incentivise businesses to make the switch to cleaner vehicles”.

Supporters of the exemption have said waiving the fees for electric vehicles played “a fundamental role” in easing the cost of investing in environmentally friendly fleets and the prospect of it ending left them “deeply troubled”.

The letter adds: “Many of us have taken on debt to invest in our children’s future and in the air that we breathe. If this plan goes ahead then it will bring an astronomical cost to our businesses at a challenging time.

“Worse still, you will hamper the efforts of many businesses transitioning to cleaner transport.

“Countless business owners wish to move away from dirty diesel vans and to electric alternatives, they should be supported and not hindered doing so.”

Oliver Lord, from Clean Cities, which is leading the campaign, said: “How is it right that a dirty diesel van pays the same as a cleaner electric vehicle in the most polluted part of the UK?

“This defies logic and the best international practice. Now is the time for the mayor to cement his efforts for change by maintaining the exemption and working on a broader package of support for green freight in the capital.”

City Hall, on behalf of the mayor, said it was also looking at initiatives including freight consolidation and cargo bike deliveries.

“We continue to work with government on national measures that could make a difference, including the extension of the plug-in van grant,” a spokesperson added.