They are also calling for a major rethink of the official approach to alternative fuels to restore confidence among fleet buyers.
Many carmakers have ploughed millions of pounds into developing alternative fuel models, particularly liquefied petroleum gas engines, while fuel providers have created a refuelling network of 1,200 sites.
But despite their efforts and initial enthusiasm from Government ministers, official support for the fuel has suddenly waned.
In the 2004 Budget, Chancellor of the Exchequer Gordon Brown announced a steady increase in duty levels for LPG.
There are also concerns about reductions in PowerShift grants available to help fleets go green.
Officially, the Government believes its policy of setting out LPG fuel duty several years in advance should make the market more confident, but that is not what most manufacturers are saying.
Paul Thomas, managing director of Ford UK, said: 'We need clear signs from Government on the future direction we should be taking, ideally across the European Union.
'The danger is if there is a change of policy, then we could have spent months or years going up a blind alley.'
Peter Milner, general manager, fleet sales, at MG Rover, said: 'Manufacturers which offer LPG conversions are being hampered by misleading messages from the Government.
'If the Government was to issue a clear statement on the benefits of LPG and its future plans, people would take it more seriously.'
Ian Rendle, corporate sales manager for Volvo, said: 'I would say we have been affected by Government indecision and lack of consistency, both on the fuel and the grants available.
'The stop-start approach of the grants scheme is not helping either. To get fleets interested, we need long-term stability for both fuel and grants.'
Last year, bi-fuel sales at the firm dropped to 522 vehicles compared to 689 in 2002. So far this year, 47 have been registered.
However, despite the concerns, there is still a commercial benefit to offering an LPG alternative in a manufacturers range.
At Mitsubishi, 45% of retail sales of its new Outlander were LPG in March, when the car first became readily available.
A spokesman for the Energy Saving Trust said: 'Our role is forming a market place and acting as an independent and trusted adviser.'
Survey looks at green grants
A MAJOR consultation is expected to be launched this month to determine the future direction of green vehicle grants for fleets.
The survey, carried out for the Energy Saving Trust in partnership with the Department for Transport, will help reshape schemes including PowerShift, which provides grants for fleets to purchase clean fuel vehicles.
Richard Tarboton, head of TransportEnergy, said: 'The consultation will look at the marketplace and the future. We will be looking at the size of the grants available and what needs to be supported and where the funds should be spent.
'We will be talking to all the stakeholders involved and see what recommendations they come back with.'