New UK car registrations fell 4.4% in September from a year earlier, according to the motor industry.
That made it the worst September this century in what is normally the industry's second most important month.
There were just 328,041 new registrations in the month, said the Society of Motor Manufacturers and Traders (SMMT).
The car sector has been hit hard by the coronavirus pandemic, which closed factories and showrooms.
September is normally second to March as the industry's most important sales month, because licence plate changes typically prompt a spike in demand.
But the SMMT said last month saw the lowest volume in new cars since the current licence plate system began in 1999.
"During a torrid year, the automotive industry has demonstrated incredible resilience, but this is not a recovery," said SMMT chief executive Mike Hawes.
"Unless the pandemic is controlled and economy-wide consumer and business confidence rebuilt, the short-term future looks very challenging indeed."
The SMMT said the relaxation of Covid lockdown restrictions from June had seen consumers return to showrooms and factories restart production lines, after one of the bleakest periods in the sector's history.
But it added that the market faced "continued pressure", including Brexit uncertainty and the threat of tariffs. while the shift towards zero emission-capable vehicles required huge investment.
"Additionally, consumer and business confidence is threatened by the forthcoming end of the government's furlough scheme, an expected rise in unemployment and continuing restrictions on society as a result of the pandemic," the SMMT said.
The SMMT added: "With little realistic prospect of recovering the 615,000 registrations lost so far in 2020, the sector now expects an overall 30.6% market decline by the end of the year, equivalent to some £21.2bn in lost sales."
However, not all carmakers had a bad month.
Historic UK car marque MG, now Chinese-owned, said it had notched up 3,668 sales in September 2020 - 169% up on the same month in 2019.
source: BBC News