Sir Liam: 'I want to see fitness tests in schools'
Fitness tests should be introduced for children in secondary schools to help make people more active, the chief medical officer for England says.
Sir Liam Donaldson said routine 'bleep tests' could help reverse the declining fitness levels of children.
In his annual report, he said inactivity was rife - research shows the majority of people do not do the recommended levels of activity.
Sir Liam said this was putting people at risk of a range of diseases.
Activity has been shown to reduce the chances of illnesses such as heart disease, diabetes and some cancers.
It can also help improve mental health.
But Sir Liam's report, the last before he steps down in the summer after 12 years in the post, said too many people were failing to heed the warnings.
Surveys shows that more than 60% of adults do not do the recommended 30 minutes of activity five days a week.
For children, who should do an hour a day, the numbers are even higher at over 70%.
Sir Liam said: "Inactivity pervades the country. It affects more people in England than the combined total of those who smoke, misuse alcohol or are obese.
"Being physically active is crucial to good health. If a medication existed that had a similar effect on preventing disease, it would be hailed as a miracle cure."
Sir Liam said there were a number of steps that need to be taken to tackle the problem.
Most children are not doing enough physical activity
He suggested piloting bleep tests - where pupils carry out a series of shuttle runs - could make an important contribution in identifying where there were particularly bad problems.
Some schools still use the tests, but Sir Liam said it would need to become a routine programme in the trials in a similar way to the national weight measuring scheme.
It comes after previous research has suggested child fitness levels are falling by 9% a decade.
Sir Liam also called for support to be given to grandparents to help them become "health mentors".
His report said they were playing an increasing role in their grandchildren's lives but less than 5% played sport with them.
And Sir Liam also called for more consistent advice on recommended activity levels - the guidelines differ slightly across the UK.
His report also highlighted the problems people with rare diseases face.
A rare illness is one which affects fewer than five in every 10,000 people.
There are currently more than 6,000 such conditions affecting about 3m in England.
Sir Liam said misdiagnosis was often a problem with some people facing delays of up to 30 years from the first symptoms appearing.
He said more specialists needed to be trained and a government tsar appointed to champion the cause.
Dr David Vickers, of the Royal College of Paediatrics and Child Health, gave the report his backing.
"We recognise that inactivity in adults begins in childhood - an issue on which we have strongly campaigned."
The Department of Health said it would consider the recommendations.