The market remained strong compared with previous Septembers, up 19,606 units on the average total since the twice yearly plate change was introduced in 1999.
SMMT chief executive Christopher Macgowan says: "We need to be clear that demand for new cars remains strong. However, it is inevitable that a plateau would be reached after three record years and it appears that further growth this year is unlikely. The impact of rising interest rates and the effect on consumer demand from a cooling housing market has, no doubt, accelerated the slowdown in the new car market in the last few months.
"In an intensely competitive market, new car buyers can expect some great deals in the next few months and despite modest reductions in volume, we still expect to be close to last year's record when more than 2.57m new cars were registered in the UK."
September typically accounts for 17% of the full year market and the performance of the '54' plate was broadly in line with expectations.
The Vauxhall Corsa took top honours in the registration charts for the second successive September. Ford's Focus tops the year-to-date list and the diesel market. The Corsa moved the Focus from the top spot for only the second time in 53 months.
The Ford Ka made it into the top ten for the first time this year, while the Fiesta moved into third place in the year-to-date listing.
Demand for sports cars, MPVs, luxury saloons and dual purpose 4x4s rose in September by 16.8, 11.3, 10.6 and 4.5% respectively.
The diesel market has risen every month in the past four years.
The growth rate has shown virtually no sign of easing. Every month of 2004 has seen double digit growth, with the September market up 17.4%. This follows a 21.1% increase 12 months ago.
Diesel penetration hit an all time high of 32.9% in September. Volumes have risen by 107,549 units over the year-to-date.