Loss of smell, fatigue, problems sleeping, mood changes, constipation and unexplained pain can all be early indications of the disease, best known for the tremors it can cause.

Although a well known disease, GPs only see patients with it rarely, according to The Cure Parkinson's Trust.

As a result they tend to be unaware that more than 90 per cent of people with Parkinson's experience such "non–motor symptoms".

Professor Ray Chaudhuri, a consultant neurologist at Kings College London, said: "Generally speaking most GPs will only see one or two people with Parkinson's in many years of work.

"Their general knowledge of the condition tends to be very poor, and it's very unlikely they would know the fine details."

This meant most people were only diagnosed late in the course of the disease, when the brain had already deteriorated considerably, he said.

About 1 in 500 people in Britain – some 127,000 –suffer from Parkison's, mostly over 50.

It is caused by nerve cell loss in the brain that result in low levels of a neurotransmitter called dopamine.

Those with advanced disease are often unable to walk unaided, have stiff muscles, and can have considerable trouble eating and speaking.

The neurological condition is progressive and their is no cure, although drugs do exist that can control some of the symptoms.

Prof Chaudhuri said that, while individually problems like loss of smell or sleep disturbance were highly unlikely to be signs of Parkinson's, if they occurred together as a "constellation of symptoms" they could be indicative.

He said that even in those already diagnosed with Parkinson's, non–motor symptoms were not identified in about 60 per cent of neurology consultations.

The Cure Parkinson's Trust has helped develop a 'personal wellbeing map' to help sufferers and doctors understand the complex condition better.

Meanwhile, another charity is announcing a landmark study to come up with a blood test to identify the disease early.

Parkinson's UK wants to enroll 3,000 volunteers in its £1.6 million 'Tracking Parkinson's' project, that hopes to tease out the genetic and environmental factors that trigger the disease.

They want to enroll 2,000 diagnosed in the last three years, 500 early developers – diagnosed before their 50th birthdays – and 500 siblings of Parkinson's patients.

Dr Kieran Breen, director of research and innovation at Parkinson's UK, said: "By the time a person is diagnosed with Parkinson's they've usually lost a lot of capacity.

"What we want to be able to do is identify them at the earliest possible stage, so we can get in and treat the underlying cause of the disease, rather than just the symptoms."

He explained the volunteers would all have their genomes sequenced, to look for responsible genes, blood samples taken and followed for three to five years to see how they changed.

Enrolling brothers and sisters, who he said had an "almost identical genetic makeup" would enable them to examine what lifestyle factors were important.

He said the "ideal conclusion" of the project would be to identify a series of "biomarkers" capable of spotting very early Parkinson's with a high degree of accuracy.

He said: "By the time we have got accurate biomarkers, we will probably have drugs that can halt the progress of the condition."

But it was "impossible to say" how long it might take to reach a suitable set of biomarkers capable of forming the basis of a reliable, accurate screening programme.

Potential biomarkers, known to be raised in those already diagnosed, will be the first candidates, he said, but these might not show in those with very early disease.

He said it was also likely a number would be needed, not just one, because Parkinson's appeared to be a range of closely–related conditions.