Cancer Research UK said this estimate, using a new calculation method, replaced a forecast of more than one in three people developing the disease.
It said longer life expectancies meant more people would be affected.
But it was not inevitable and improving lifestyle, such as losing weight and quitting smoking, could have a major impact, the charity added.
The good news is cancer survival figures are also rising.
The seemingly sudden jump in diagnosis estimates is down to researchers developing a more sophisticated and accurate method for analysing the risk of cancer.
However, both the new and old methods show the same long-term trend - a rise in the lifetime risk of developing cancer.
Nearly 54% of men will develop cancer, compared with just under 48% of women, the figures indicate.
Food pipe tumours
Fewer deaths from heart disease and infections mean more people are living long enough to develop cancer.
But lead researcher Professor Peter Sasieni, from Queen Mary University of London, said: "It isn't inevitable.
"There is quite a lot we can do to prevent cancer and hopefully in many years' time I'll have been proven completely wrong."
He is referring to lifestyle factors including obesity, red meat consumption and smoking that increase the odds of a tumour developing.
Lung cancer cases are still increasing in women
Breast cancer is likely to remain the most common cancer among women
He told the BBC that a healthy lifestyle could lower the lifetime risk from 50% to 30%.
Breast and prostate cancers are likely to remain the most common cancers in women and men respectively.
However, some cancers are rapidly becoming more common.
Tumours in the food pipe, caused by acid reflux in obesity, are being seen more often in clinics.
Head and neck cancers caused by the human papillomavirus are increasing and oral sex is thought to be behind the rise.
Dr Harpal Kumar, the chief executive of Cancer Research UK, said: "We have reached what many would regard as an important milestone.
"We need to plan ahead to make sure the NHS is fit to cope, if the NHS doesn't act and invest now, we will face a crisis in the future - with outcomes from cancer going backwards."'