GPs should refer to a specialist if symptoms strongly indicate lung cancer even if a chest X-ray is negative.
Professor Michael Feckl, a consultant oncologist at Cancer Research UK, said: 'You can't rely on a chest X-ray to exclude a diagnosis of lung cancer.
'If the chest X-ray is normal but you can't explain what's going on, there needs to be low threshold for referral.'
Dr Steve Holmes, chairman of the General Practice Airways Group, agreed: 'If an X-ray does not show cancer but the case is still suspicious, GPs should refer in the quickest way possible.'
The advice follows research showing that chest X-rays for lung cancer come back negative in a quarter of patients who have the disease.
Ten per cent of X-rays from 164 patients who were diagnosed with a primary lung cancer a year later came back normal and a further 13 per cent were abnormal, but had no sign of malignancy.
Lead researcher Dr William Hamilton, from the primary healthcare unit at the University of Bristol, said that despite the findings, chest X-rays remained 'the most valuable first step' in diagnosing lung cancer.
But he added: 'GPs have a good knack for spotting the ill patient. It's important for them to keep hold of that skill.'
He said that GPs unsure whether to refer should repeat the chest X-ray with a lateral, and then if still unsure refer.
NICE clinical guidelines on lung cancer state that GPs should send patients for a chest X-ray if they have haemoptysis or other unexplained or persistent symptoms such as cough, shoulder pain or dyspnoea. Urgent referral to a specialist is needed after a positive X-ray, and for those with a negative X-ray if there is a high suspicion of lung cancer.
Br J Gen Pract 2006; 56: 570-3