Introducing March and September numberplate changes was intended to spread car sales more evenly over the year.
But after just two years of the new scheme, there are concerns fleets are focusing on March as the key time for taking on vehicles, leading to claims that it has become the 'new August'.
Up to 420,000 vehicles are expected to be registered during March, more than a fifth of annual sales, and although September sales are normally just as high, industry experts fear this year it might be a 'damp squib'.
The are also concerns that two plate changes a year accelerate the speed at which cars devalue, pushing residual values to new lows.
Martin Ward, national research manager for used car price experts CAP, said: 'The twice-yearly numberplate system is doing more to devalue used cars than anything else.
'Manufacturers I have spoken to have said that more than 400,000 units will be achievable in March, which makes it the equivalent of August under the previous registration system. September is just a damp squib by comparison.'
In September 2002, new car sales were 432,661, 24% down on September 2001. By comparison, in March 2002, new car sales were 423,723, an increase of 3.8%. In contrast, August sales under the old numberplate system were regularly more than 500,000.
Manufacturers also believe March is becoming the most important month in the new car sales calendar.
A spokesman for Mitsubishi said: 'With more emphasis seemingly being paid by consumers in having a distinctive numberplate such as 03 for their vehicles, which denotes a year rather than part year such as 53, March could soon take over as the prime month for new cars sales rather than September.'
But the new numberplate system was defended by the Society of Motor Manufacturers and Traders. A spokesman said: 'If manufacturers are to deliver quality to customers, then it makes sense to spread sales throughout the year, rather than try to get them all into one month.'