Medics and campaigners have warned the public to pay attention to changes to their fingertips, as a specific alteration could be a red flag sign of lung cancer.

Well-recognised signs of the deadly disease include a persistent cough and breathlessness, but experts have now flagged a lesser-known, unusual symptom that affects the fingers.

Swelling in the fingertips, known medically as finger clubbing, is a known sign of the disease. 

It's thought to be a result of a number of factors, including substances released by certain lung tumours that inflame the bones in the fingers, and harmful chemicals produced by the body that fuel cancer development.

The problem can appear as swollen fingers, as well as a change in the shape of the nail. 

Now, a former patient whose only symptom was finger clubbing has warned others to be aware of the unusual sign and take action quickly.

'Go and see your doctor—that's what a GP is for,' Brian Gemmell, a fitness instructor from East Kilbride, Scotland, told The Mirror. 'Go as soon as you can.'

Mr Gemell previously told The Roy Castle Lung Foundation that he suffered no cough.

 'I was feeling healthy,' he said. 'I wasn’t coughing up blood, I wasn’t breathless. My one and only symptom was clubbing of the fingers, where all your fingers swell up and when you put your fingers together you can’t see a diamond. 

'That was my only symptom. Straightaway, my GP had an idea of what it was. He sent me straight for a chest X-ray and referred me to a respiratory consultant.' 

Mr Gemell then underwent surgery and other treatment, and now lives with stage three lung cancer. 

After his surgery he re-trained as a personal fitness instructor, working predominantly with cancer patients. 

Experts recommend using an at-home screening tool known as a Schamroth window test to check for finger clubbing.

It involves pressing two opposing fingers back-to-back against each other.

If you can't see a diamond-shape window of light between the nail beds and the nails of the two fingers, this could be an indication of finger clubbing.

According to a 2012 study by Dr Malay Sarkar, an India-based lung cancer specialist, lung cancer is the primary cause of clubbing, contributing to nearly 90 per cent of cases.

Meanwhile, he estimates that 5-15 per cent of people with lung cancer develop nail clubbing.

According to Dr Sarkar's study, clubbing seems to occur in different stages, starting at the base of the nail, which becomes soft and spongy.

It usually affects both hands, causing redness of the skin around the nail—known as erythema.

The angle between the nail bed and the skin just below the cuticle then becomes noticeably bigger, causing the nail to curve more than usual.

At this point, Dr Sarkar notes, the nail and the surrounding skin will look shiny and the nail will have ridges along the length of it.

Eventually, the finger develops a clubbed appearance. While the condition typically takes years to develop, it can happen quicker if there is certain complications such as a lung infection, Cancer Research UK warns.

In later stages, extra areas of bone can form on the finger joints, wrists and even ankles—a condition which is known as hypertrophic pulmonary osteoarthropathy (HPOA), and is often mistaken for arthritis.

Not everyone with finger clubbing will have lung cancer, according to Cancer Research UK.

But the charity advises those who think they may have it to contact their GP.

The warning comes as Figures from Cancer Research UK show cases of all types of lung cancer have risen by 130 per cent in young women over the past few decades.

This makes them the fastest growing group at risk of the disease — although over-70s are still most likely to be diagnosed.

In contrast, rates among men of the same age have barely budged from the early 90s, for reasons that medics say are simply unknown.

Experts have warned against viewing the deadliest form of cancer as a disease that predominately affects old people—as it has previously been seen—as cases rise among never-smokers worldwide.

According to the International Agency for Research on Cancer, lung cancer in people who have never smoked is now the fifth highest cause of cancer deaths globally.

Symptoms of the cancer include a persistent cough that doesn't go away after three weeks, repeated chest infections, coughing up blood, pain while breathing, persistent breathlessness and fatigue, and unexpected weight loss.

Other, more unusual signs of lung cancer include change in the appearance of your fingers, difficulty or pain swallowing, wheezing, changes in your voice, and swelling of the face or neck.