After April 1, TransportAction will reduce the maximum level of PowerShift grant available to 60% (from 75%) of the additional costs of buying or converting 'greener' vehicles.
The move is expected to free-up grant funds for a widening of the scheme announced last November to include the conversion of cars up to five years-old.
To qualify for a full grant, a vehicle must be under one year-old and produce emissions when converted that are more than one-third cleaner than Euro IV standards.
David Hepworth, customer services manager of TransportAction, said: 'Business fleet managers are looking closely at liquefied petroleum gas following the introduction of the new CO2-based company car tax system, which favours the lower CO2 emissions from LPG-fuelled vehicles over their petrol equivalents.'
TransportAction will accept completed application forms submitted by the end of March for the full 75% grant, all other criteria being fulfilled.