More than 460,000 new cars were sold last month - the highest September figure on record.
There were 462,517 registrations, 8.6% more than the same month last year, said the Society of Motor Manufacturers and Traders (SMMT).
More than two million new cars have been sold since the start of the year, the best performance in over a decade.
Car sales have now increased for 43 months in a row, said the SMMT.
There was increased demand across all fuel types of vehicles and from private, fleet and business buyers.
A new number plate means September is traditionally one of the biggest months of the year for car sales.
SMMT chief executive Mike Hawes said: "September is traditionally one of the year's biggest months for new car registrations, and last month set an autumn record.
"With plenty of attractive, affordable deals available on the new 65 plate, Britain's car buyers - whether private, fleet or business consumers - were busier than ever. The market reached pre-recession levels some time ago, and we anticipate some levelling off in the coming months.
"It is too early to draw conclusions, but customer demand for diesel remained strong, accounting for one in two cars registered."
1. Ford Fiesta - 22,807
2. Vauxhall Corsa - 15,670
3. Ford Focus - 14,258
4. Volkswagen Golf - 13,603
5. Volkswagen Polo - 11,026
6. Nissan Qashqai - 10,119
7. Mini - 8,754
8. Vauxhall Astra - 7,920
9. Vauxhall Mokka - 7,670
10. Fiat 500 - 7,460
Year to date
1. Ford Fiesta - 108,054
2. Vauxhall Corsa - 72,835
3. Ford Focus - 69,053
4. Volkswagen Golf - 59,200
5. Nissan Qashqai - 49,715
6. Volkswagen Polo - 45,537
7. Vauxhall Astra - 42,687
8. Audi A3 - 38,253
9. Mini - 35,679
10. Mercedes-Benz C Class - 35,505
Ian Gilmartin, head of retail and wholesale at Barclays, said: "September is always a crucial month for the motor industry and, following a difficult couple of weeks, this positive data will rightly be welcomed. However, we need to ensure that increasing registrations really do translate to good results in the showrooms and forecourts.
"Margin pressure is a growing concern for some dealers, and finding the right formula to ensure that the whole industry continues to succeed is going to be vital in the final part of 2015."