At the end of August, the £12.50 a day charge for the most polluting vehicle will be extended to cover the whole of London.
The Diamond Centre is right on the edge of Sutton and is a 30-minute walk from the closest train station.
It provides therapeutic time with horses for adults and children with disabilities.
Chair of the board of trustees Claire Snowdon said many of the centre’s disabled riders simply won’t be able to get to their sessions in Woodmansterne Road, Carshalton, once ULEZ is expanded.
Ms Snowdon said: “There is one rider who has ridden with us for 35 years who will not qualify for any exemptions and not be able to afford to replace their car so will not be able to come any more.
“More than 20 per cent of our current users don’t have compliant cars and the majority of them don’t qualify for any exemptions.
“We are surrounded by farms and parkland and there is no public transport here so our users are completely reliant on private vehicles to get here. For some of the riders it is their only chance of independence it is a very social experience and they get exercise from it.
“It is good for core strength and balance as well as motor skills, communication and confidence.
“There are so many benefits from riding horses if you have a physical disability or learning disability.”
The charity offers a range of services, including riding lessons, carriage riding as well as horse sensory sessions called tea with a pony for people with dementia.
As well as losing customers who benefit from the service, Claire said there is the risk of losing some of the 330 volunteers.
Its location on the edge of Sutton means many volunteers and customers travel from Surrey, meaning their non-compliant cars will not be eligible for TfL’s scrappage scheme.
Ms Snowdon added: “We really don’t feel like we are in London here, we are rural.
“The fact that there is absolutely no public transport out here means people have to drive, there is no alternative.
“You can see how it would work in the middle of London or even in built up areas that are well served by public transport.”
Christina Calderato, TfL’s director of strategy and policy said: “The London-wide ULEZ is vital in tackling the triple challenges of air pollution, the climate emergency and congestion, and will ensure millions more people can breathe cleaner air. We are already seeing 90 per cent of cars across outer London on an average day complying with the standards and expect this to be even higher when the scheme goes live.
“In order to support smaller businesses, sole traders, charities, Londoners on lower incomes and disabled Londoners in replacing their older, more polluting vehicles with greener options, the Mayor has launched London’s biggest ever scrappage scheme worth £110m.
“The scheme also includes grace periods for disabled people up to October 2027 and also covers a nominated driver, regardless of whether they live in London.
“We are happy to meet with the riding centre and discuss the options available to them.”