Average CO2 emissions have climbed almost three per cent to 124.5g/km, the Society of Motor Manufacturers and Traders (SMMT) has warned.
The rise has been blamed on a diesel sales slump and experts say if CO2 emissions continue to rise it could have detrimental effects on the environment and also see countries miss their targets for slashing emissions.
Just over 750,000 new UK diesel cars were registered in 2018 compared to 1.06 million in 2017.
The overall market percentage of diesel cars also slumped from 42 per cent to under 32 per cent over the space of 12 months.
December marked the 21st consecutive month of decline for the fuel type – despite new emissions tests showing diesels deliver in the real world.
The SMMT states that diesels are on average, 15-20 per cent more efficient than petrol equivalents.
Over the course of last year, petrol car registrations increased by 100,000, with the fuel type now occupying a 62.3 per cent market share.
Mike Hawes, SMMT chief executive, said, “A second year of substantial decline is a major concern, as falling consumer confidence, confusing fiscal and policy messages and shortages due to regulatory changes have combined to create a highly turbulent market.
“The industry is facing ever-tougher environmental targets against a backdrop of political and economic uncertainty that is weakening demand so these figures should act as a wake-up call for policy makers.
“Supportive, not punitive measures are needed to grow sales, because replacing older cars with new technologies, whether diesel, petrol, hybrid or plug-in, is good for the environment, the consumer, the industry and the exchequer.”