Driverless cars could increase congestion on some UK roads for several years, according to a Department for Transport study.

Delays on motorways and major roads during peak periods are expected to rise by 0.9% when one in four cars are automated, researchers found.

Early models of driverless cars are expected to operate more cautiously than regular vehicles, resulting in "a potential decrease in effective capacity and a decline in network performance", the report warned.

The analysis suggested that a reduction in congestion may not be achieved until automated vehicles make up 50% to 75% of the fleet.

Steve Gooding, director of motoring research charity the RAC Foundation, said: "There's a prize to be had in terms of swifter, safer journeys, but the transition to that world will be challenging.

"There are around 32 million conventional cars on the UK's roads - as driverless cars come in, traffic flow could initially get worse rather than better, potentially for many years.

Much will depend on how an autonomous car's parameters are set and just how defensively these vehicles will be programmed to drive

"Much will depend on how an autonomous car's parameters are set and just how defensively these vehicles will be programmed to drive."

The DfT's study - which involved using computer software to create virtual models of different parts of the road network - found there could be a 40% cut in peak time delays on motorways and major roads once if vehicles are driverless.

Researchers believe urban roads could see average delays cut by 12.4% if just 25% of cars are automated.

The DfT said the report paves the way for further trials and research into driverless vehicles to ensure their development is "safe and beneficial for all".

Transport minister John Hayes said: "This exciting and extensive study shows that driverless cars could vastly improve the flow of traffic in our towns and cities, offering huge benefits to motorists including reduced delays and more reliable journey times."

He said driverless cars are just one example of new technologies that could transform travel in the future, particularly for people with reduced mobility.