The UK government’s weakening of rules on electric car sales is likely to result in significantly more carbon emissions as sales of plug-in hybrids rise, according to analysis.

Relaxed rules could result in as many as 500,000 additional plug-in hybrid electric vehicles (PHEVs) on UK roads by 2030, according to modelling by T&E, a campaign group on transport and the environment. PHEVs combine a small battery with a polluting petrol engine and tend to be much more profitable than pure battery electric cars.

Experts said the findings suggested emissions would rise significantly because of the Labour government’s policy. The transport secretary, Heidi Alexander, last month told parliament the weaker rules, known as the zero emission vehicle (ZEV) mandate, would cause a “negligible change to the carbon emissions” from cars sold in Britain.

The government last month weakened the rules, which force carmakers to sell an increasing number of battery electric cars each year in order to meet the UK’s climate targets. Keir Starmer argued that the government needed to help struggling carmakers, allowing more “flexibilities” that mean carmakers can sell fewer electric cars and avoid steep fines.

Tim Dexter, a vehicle policy manager at T&E UK, said: “The government’s changes will allow car manufacturers to sell more plug-in hybrids and for longer. Instead of accelerating the transition to zero-emission cars, the industry can maximise sales of lucrative PHEVs, which emit far more than claimed.”

In some years the sale of PHEVs could almost double, according to the analysis. PHEV sales are now expected to peak in 2028, at 280,000 a year, up from a previous forecast of 180,000 a year in 2025. Last year the industry sold 167,000 PHEVs, up 18% on the previous year.

Colin Walker, the head of transport at the Energy and Climate Intelligence Unit, a campaign group, said: “Not only will this result in more CO2 being emitted in the midst of a climate crisis, it will hit the UK’s drivers in their wallets.”

He said the UK government was “distracting manufacturers from making the shift to building the EVs on which the UK car industry’s future depends”.

Alexander’s emissions claim appeared to be based on government policy that has been criticised by experts for relying on unrealistic figures. Crucially, the government’s analysis assumed PHEVs will continue to achieve the same share of the market as 2024 – despite generous emissions assumptions giving manufacturers a big incentive to sell more hybrids.

PHEVs have a polluting petrol engine but can make big emissions savings by running on a small rechargeable battery whenever possible. In an ideal world, PHEV owners would use the battery for most small trips, only using the engine for longer journeys. However, a number of studies have found that PHEV drivers do not reliably charge their vehicles, meaning the cars emit much more in the real world.

The EU is updating official figures to reflect this disparity. By 2028, average PHEV emissions will be assumed at 109 grams of CO2 per kilometre, up from the 32g average currently used. However, the UK government will continue to use the 32g average despite the Department for Transport in December acknowledging that PHEVs have “much higher CO2 emissions in the real world”. That will give carmakers a strong incentive to sell more PHEVs, as they will gain credits for lower emissions, meaning they have to sell fewer electric cars.

Ben Nelmes, the chief executive of New AutoMotive, a thinktank that has closely followed the ZEV mandate, said: “The government – and anyone who has bought a car – knows that manufacturer claims about MPG ratings should be taken with a large helping of salt. Yet ministers have decided to stake the UK’s largest climate policy on the accuracy of these ratings.

“That will sound naive to most people. We should not be surprised to see UK emissions fall more slowly as a result of recent changes to electric car targets.”

T&E’s Dexter said: “The government can still mitigate some of the extra emissions caused by this loophole by officially recognising the real-world emissions of PHEVs. That will account for hybrids’ true climate impact and make buyers aware of their extra cost at the pump.”

A DfT spokesperson said: “Our recent changes strike a practical balance – giving manufacturers flexibility to sell plug-in hybrids until 2035, while sticking to our commitment to the 2030 phase-out of new petrol and diesel cars.

“These changes are a practical and balanced approach which will have a minimal impact on emissions while supporting drivers, giving long-term certainty to the industry and protecting jobs across the UK.”