There were 142,322 new car registrations in April 2012 - a 3.3 per cent rise on the April 2011 figure, the SMMT said.
The April 2012 increase was largely driven by a big rise in private sales which were almost 15 per cent up on the same month last year.
Last month's total new-car sales took the year-to-date figure to 705,878 - a 1.4 per cent rise on the January-April 2011 total.
SMMT chief executive Paul Everitt said: "A strong April new car market has raised confidence across the UK motor industry.
"We are seeing a steady increase in consumer confidence with growth in private demand, boosted by the arrival of a raft of new products.
"We have increased our 2012 full-year forecast to 1.95 million units, putting the market marginally ahead of 2011, indicating a period of relative stability."
The growth in registrations came on top of a weak 2011 performance, SMMT said. Total registrations for January-April 2012 are around 18 per cent below the 2007 pre-recession level, but 15 per cent above the low of 2009, it said.
The market share for petrol cars grew in April 2012, although over the first four months of the year diesel and alternatively-fuelled car registrations have shown volume and market share growth.
Small car demand (mini and supermini) rose strongly in April, as too did volumes of dual-purpose segment cars. The executive and sports car markets also rose.
These were the best-selling models in April 2012:
1. Ford Fiesta
2. Vauxhall Corsa
3. Ford Focus
4. Vauxhall Astra
5. BMW 3 Series
6. Volkswagen Golf
7. Volkswagen Polo
8. Nissan Qashqai
9. Vauxhall Insignia
10. Fiat 500