In its monthly analysis, Cap Red Book said: 'Gas-powered engines from some manufacturers fail to meet the emissions criteria for receiving a 100% discount on the congestion charge in central London when it goes live on February 17.
'It seems ironic because even some of the petrol engines produced by the same manufacturers are now only just short of the PowerShift band four level (the criterion vehicles have to meet to qualify for the 100% discount).
'But the worrying fact is that vehicles already in the marketplace will be worthless if intended solely for the purpose of driving in and out of London.'
CAP added: 'Therefore, although prices have been holding up on alternative fuel vehicles, this news could have a disastrous effect on the used market and residual values.'
Manufacturers are struggling to hit band four emissions standards for LPG-powered vans because their petrol engines are relatively clean already. To achieve a band four rating on the PowerShift register, a van must have a 40% emissions reduction over the Euro IV standard.
CAP says fleets should contact their local garages to find out if their vans meet the band four emissions standards, and warns that it is a potential minefield for the trade: if customers buy an alternative fuel van and then get charged for running it in London they will, according to CAP, 'beat a path to your door demanding blood'.
Ironically, CAP adds, less clean vehicles which have had retrofits of alternative fuel systems could qualify for the congestion charging discount, despite having their manufacturer warranty invalidated by the work. This is because they have achieved a 40% reduction in emissions from their original levels.
At present, there are only 13 car-derived vans and 17 vans that qualify for band four on the PowerShift register.