The data shows that the problem of "head-nodding" is a common occurrence – this happens when someone nods off for between two and 30 seconds, often without realising that they have been asleep. Yet nodding off for just a few seconds at the wheel can be fatal: if you are driving on a motorway at 70mph and nod off for six seconds you would travel nearly 200 metres, which could take you across three lanes of traffic and down an embankment onto another road or train track.

Experts estimate that tired drivers cause one in five fatal crashes on motorways and other monotonous trunk roads. Crashes caused by drivers falling asleep at the wheel tend to be high-speed crashes, because drivers do not brake before crashing, so the risk of death or serious injury occurring is greater than in other types of crashes.

Many drivers are guilty of risk behaviour that can lead to tiredness. The survey of 1,000 drivers found that one in four admitted to embarking on a journey when they already felt drowsy. The vast majority (86%) are also failing to follow best practice advice on dealing with tiredness at the wheel, by stopping somewhere safe for a nap. More than a quarter (29%) put their own and others' lives on the line by continuing their journey after they notice the first signs of drowsiness.

In addition, one in seven drivers surveyed (13%) reported suffering from a health condition such as sleep apnoea that makes them tired during the day. Sleep apnoea can cause daytime sleepiness, and in some cases can cause the sufferer to fall asleep without warning.

The research results, which were conducted along with the Cambridge Weight Plan, are being presented today at a Parliamentary reception attended by MPs, fleet and road safety professionals and civil servants.

Julie Townsend, Brake's campaigns director, said: 'Tiredness at the wheel kills. Driving a vehicle is a huge responsibility that must be taken seriously. That means stopping when we feel drowsy and certainly never starting a journey tired. It's a matter of life and death. We still have widespread misunderstanding of how to prevent driver tiredness, and ignorance about factors like sleep apnoea, a condition that can be treated. These messages still need to get through to the public, which is why we are calling for renewed efforts from the Government to tackle this issue urgently.'