Demand for new cars fell by 3.5% last month, figures show.
Almost 235,000 new cars were registered in June compared with just over 243,000 during the same month in 2017, the Society of Motor Manufacturers and Traders (SMMT) said.
Registrations are down 6.3% this year compared with the first six months of last year.
Sales of petrol models increased by 12.3% last month, while diesels fell for the 15th consecutive month, down 28.2%.
Demand for alternatively fuelled vehicles such as hybrids and pure electrics increased by 45% to take a market share of 6.6%.
There has been growing concern about the impact of diesel car emissions on air quality and uncertainty about what taxes and restrictions will be introduced on them.
SMMT chief executive Mike Hawes said: "Despite a rocky first six months for the new car market, it's great to see demand for alternatively fuelled vehicles continues to rise.
"Given these cars still represent only one in 20 registrations, however, they cannot yet have the impact in driving down overall emissions that conventional vehicles, including diesels, continue to deliver.
"Recent Government statements acknowledging the importance of petrol and diesel are encouraging.
"However, we now need a strategy that supports industry investment into next-generation technologies and puts motorists back in the driving seat, encouraged to buy the car that best suits their needs - whatever its fuel type."