Many people are visiting their doctor with symptoms they do not realise are caused by dehydration, GPs have warned.

A survey of GPs found that about one in five (21%) consultations involved patients whose primary cause of concern was fatigue and tiredness, which is often caused by not drinking enough fluid.

While official guidelines suggest adults should drink between 1.6 and two litres a day, just 4% of the 300 GPs questioned said they believed their patients are aware of how to hydrate properly and research suggests that many people drink just one glass of water - or even less - each day.

They argued that while many complain they are unable to get an appointment to see their GP, their time is being taken up by a problem that can easily be resolved.

Dr Roger Henderson, a GP and adviser to the Natural Hydration Council, said: "I see many people in my surgery who are feeling tired all the time.

"There are, of course, several reasons that could be causing this but a surprisingly common cause is that they are dehydrated.

"Many of my patients do not drink enough fluid each day and only believe they are dehydrated when they start to feel thirsty.

"Yet other symptoms of dehydration appear before this, including fatigue and tiredness, headaches and poor concentration.

"When people start to ensure they are staying fully hydrated they are often surprised at how much better they feel, both physically and mentally.

"It is, therefore, vital that drinking enough water becomes part of our daily routine.

"What this study shows is that as well as contributing hugely to the nation feeling tired all the time, dehydration and its effects is adding to the numbers of people trying to get a GP appointment at a time when the NHS is under immense pressure."