Family and friends could be doing more harm than good by telling people with cancer to "take it easy", a British charity says.
Macmillan Cancer Support says family and friends have a crucial role in supporting people to take exercise or to become more physically active.
A poll of more than 1000 cancer patients found 60 per cent would do more exercise if family and friends kept them company.
This had a stronger effect than factors such as cheap gym membership or advice on fitting more activity into daily life.
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Staying active has been shown in studies to help people deal with the fatigue, depression and heart damage caused by treatments such as radiotherapy or chemotherapy.
Exercise also cuts the chances of the disease spreading and returning, and the chance somebody will die from their cancer.
For example, breast cancer patients can cut the chance of recurrence and of dying from their disease by up to 40 per cent if they do regular exercise that increases their heart rate.
The survey of people living with cancer found 25 per cent had not done any physical activity that had raised the heart rate in the last seven days.
A fifth also did not feel confident about becoming more active.
"Being physically active is a hugely important part of someone's recovery from cancer, and family and friends must make sure that no one faces this often long, hard journey alone," said Lynda Thomas, chief executive of Macmillan Cancer Support.
"It's natural to tell people to take it easy when they have cancer, but we know that this isn't the best thing for many people.
"By encouraging someone with cancer to get active, family and friends can play an invaluable part in helping them get their life back."
