Around 1.7 million people in England could be living with heart disease, lung cancer or lung disease without knowing it, Public Health England (PHE) is warning.
The watchdog said anyone with an ongoing cough or who gets out of breath more easily should visit their GP as soon as possible.
Figures suggest around 80,000 people in England have undiagnosedlung cancer.
A million people have undiagnosed chronic obstructive pulmonary disease (COPD), which includes emphysema and chronic bronchitis, and another 600,000 people are living with undiagnosed heart disease.
All three illnesses are leading causes of death, with lung cancer alone accounting for around 28,400 deaths every year.
PHE is launching the next phase of the Be Clear on Cancer campaign relating to lung cancer but also wants people with signs of lung disease or heart disease to get checked.
Experts say a persistent cough or getting out of breath doing tasks that used to be managed easily, such as mowing the lawn or vacuuming, could be a sign of lung problems.
Breathlessness is also a sign of heart disease.
The campaign is aimed at men and women aged 50 and over who are most at risk of the diseases.
Professor Kevin Fenton, PHE's national director for health and well-being, said: "The estimated number of people with undiagnosed lung cancer, lung disease or heart disease is deeply concerning.
"If diagnosed early, these diseases can be managed and treated successfully.
"This campaign will help people recognise the symptoms and encourage them to seek help, potentially saving lives from what are three of the biggest causes of death in England."
Health minister Jane Ellison said: "Sadly, diagnosis often comes too late, which can have a devastating impact on those living with any of these conditions, as well as those close to them.
"The more people we can encourage to get their symptoms checked, the more likely they are to be diagnosed earlier and treated successfully.
