Small businesses say not enough is being done to highlight the new Low Emission Zone (LEZ).
Cars and motorcycles are not affected by the scheme
From 4 February older diesel-engine lorries, coaches and other heavy vehicles must pay up to £200 to enter the zone, covering most of London.
But the Federation of Small Businesses said many of its members were still unaware of the scheme.
Transport for London (TfL) said it had run two periods of consultation and carried out a publicity campaign.
The LEZ will be enforced using fixed and mobile cameras, which will read vehicles' number plate as they are driven within the zone and check them against a database of registered vehicles.
There will be a phased introduction of the scheme from 4 February through to January 2012.
Cars and motorcycles will not be affected by the zone.
Simon Briault, from the Federation of Small Businesses, said: "The concern at the moment is that so few of our members are aware of it.
"They may find themselves driving into London and then having to pay £100 or £200 a day, or as much as £1,000 in fines, for something they know nothing about, and that would be unfair."
But TfL said: "Following two extensive periods of public and stakeholder consultation, a large-scale, multi-media public information campaign was launched in June this year and uses a combination of posters, radio and print media.
"In addition, a direct mail campaign has targeted operators of vehicles that currently do not meet the required emissions standards and have been identified as travelling within Greater London."
The TfL spokesman added: "We have also held an extensive number of road shows at service stations, transport cafes, markets and ports where we have explained the scheme and distributed leaflets."