Scientists say they have identified the earliest signs of Parkinson's disease in the brain, 15 to 20 years before symptoms appear, the BBC reports.

Scans of a small number of high-risk patients found malfunctions in the brain's serotonin system, which controls mood, sleep and movement.

The King's College London researchers say the discovery could lead to new screening tools and treatments.

Experts said larger studies and more affordable scans were needed first.

Parkinson's is a progressive neurological condition affecting about 145,000 people in the UK.

The main symptoms are shaking, tremors and stiffness but depression, memory and sleep problems are also common.

Traditionally, the disease is thought to be linked to a chemical called dopamine, which is lacking in the brains of people with the condition.

Although there is no cure, treatments do exist to control symptoms - and they focus on restoring dopamine levels.