Toyota team president John Howett is unsure Formula One teams will be able to have the Kinetic Energy Recovery Systems (KERS) ready for use in time for the start of next season.
The system, which stores energy to be used in the form of a 'boost' button, will be allowed in the sport from 2009, and some teams have already started running with it.
KERS, however, has already caused two known scares, first when it triggered an incident with steam and fumes at the Red Bull factory, and again yesterday, when a BMW Sauber mechanic was injured after receiving an electric shock when touching the team's test car.
And Howett reckons most teams are struggling with the development of the system, which he believes could not be ready in time for the start of 2009.
"The challenges are enormous for everybody," Howett told autosport.com in Germany. "We're all fighting very hard to have a raceable, safe KERS, but whether it is achievable or not has to be seen.
"I think if you read between the lines of a lot of teams I think a lot of people are struggling."
Howett believes most of the problems with the system can be solved, but he is unsure if they can be overcome to a satisfactory safety level.
"Everybody knows that the type of batteries that we're using, and the voltages involved, and the training that you have to go through, for instance, releasing a road car so that when the fire brigade arrives at an accident they know what to do, is very hard," Howett added.
"So I think a lot of these issues can be overcome. Whether they can be overcome to a satisfactory, safe level before the first race, I can't hand-on-heart tell you.
"We're working flat-out to try and do that, or to find alternatives to batteries, but I doubt whether that will be feasible for next year."
Honda team principal Ross Brawn, whose team are believed to be the first ones to have run with KERS, admitted the Japanese squad are unsure if they will start the season with the system.
"I don't know for certain," he said. "We will if it's a performance gain, and we haven't done enough work yet to know if it's a performance gain. We are scheduling to start the season with it, but we will see how it develops.
"It's rather too early to say, but the new car is designed to accommodate it and the day that it becomes a positive, net performance gain we will start to race it."
Brawn also reckons the performance advantage of KERS is currently very small.
"KERS has a performance gain but the downside is accommodating and packaging all the various elements," he added. "I think the performance gain at the moment is possibly marginal, but then we haven't necessarily explored all the potential of KERS, how it can be used.
"None of us have got systems running regularly on the track to learn how they can behave. The other thing that is clear is that the FIA have indicated that they want to increase the value of KERS and of course that for sure will increase the margins."