London – The first driverless cars could be dangerous because of the time it can take for drivers to regain manual control, scientists have warned.

In a study, it took up to 25 seconds for less attentive drivers to retake the wheel after being alerted by the car’s computer system.

The first generation of automated cars will be ‘hands-off’ for only part of the journey, needing drivers to step in for complex manoeuvres such as leaving a motorway.

However, University of Southampton research found drivers took up to 25.7 seconds, when distracted, to respond to a command to take over from the car’s computer.

It is feared this could see people swerve, make sudden lane changes or brake harshly if they do not realise in time and react.

Lead author Alexander Eriksson said: "We should give people enough time to react so they are aware of the situation, can judge the traffic and are not forced to swerve or take dangerous evasive action." The study, published in the journal Human Factors, asked 26 drivers to take part in a simulated driving task at 70mph (113km/h). The drivers’ response times were recorded as they were prompted to take over by a screen message and computer-generated female voice.

When the test subjects were given a copy of National Geographic magazine to read, their reaction times slowed considerably.

Researchers found, under non-critical conditions, drivers needed between 1.9 and 25.7 seconds to take control from automation.

They say these times should be taken into consideration by manufacturers producing the cars, rather than the average time for a person to start a manoeuvre.

Discussing swerving, Mr Eriksson said: "Such actions are acceptable in safety-critical scenarios when drivers have to avoid a crash, but could pose a hazard for road users in non-critical situations."

The first fully automated cars are expected to be on sale (abroad at least) by 2020.