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 undiagnosed lung 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.
"Diagnosis often comes too late, which can have a devastating impact"Jane Ellison, Health Minister
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