Speaking at the conference, Stewart Whyte said the Government's enthusiasm had 'gone off the boil', which was highlighted in the recent Budget.
Whyte said: 'There is a sign of clear underlying environmental issues. There are exceptions of committed fleets that do their bit for the environment but in general drivers want badge prestige and don't really care about fuel economy. 'Liquefied petroleum gas (LPG) as a mainstream fuel is now less attractive and changes to Powershift grants have also caused damage.
'The Chancellor seems to have exhausted what he can do before the general election. It is a shame as we need to maintain pressure here.'
Chancellor of the Exchequer Gordon Brown announced plans to increase fuel duty on liquefied petroleum gas last year, leading to concerns in the fleet industry that its investment in green fuels had been wasted.
In the Budget, the Government announced it will reduce the duty differential in favour of liquefied petroleum gases by the equivalent of 1p per litre for each of the next three years until 2007.
The rise is equivalent to an increase of 2.42p per litre in 2004/2005.