Women who have previously had breast cancer should not let their medical history get in the way of them enjoying exercise.
Worries that physical activity will cause or exacerbate an incurable type of painful swelling should be set aside, the National Institute of Health and Care Excellence (Nice) said.
Exercise will not prevent, cause or worsen breast-cancer related lymphoedema, Nice said. And it could also improve a patient's quality of life.
Of the 50,000 people diagnosed with breast cancer in Britain each year, around one in five will develop lymphoedema in the arm following treatment, a Nice spokeswoman said.
Lymphoedema occurs when the body's lymphatic system becomes damaged and is unable to drain fluid in the normal way. This can lead to a build-up of fluid in the arms and surrounding areas which causes painful swelling.
The condition can be caused by surgery, radiotherapy or the cancer itself and there is currently no cure.
The Nice spokeswoman said: "People with breast cancer should not avoid exercise because of groundless fears it will cause or worsen (this) type of incurable and painful swelling."
Professor Mark Baker, director of Nice's Centre for Clinical Practice, said: "Lymphoedema can have a significant effect on someone's life, making once routine tasks difficult and painful. It may also have an impact on a person's self-confidence and mental health depending on how severe the condition is.
"Our new recommendations make clear the link between exercise and breast cancer-related lymphoedema to help people manage this often distressing condition. Although exercise will not prevent it, evidence shows that it will not cause lymphoedema or make existing symptoms worse either."
Rachel Rawson, clinical nurse specialist at Breast Cancer Care, added: "Lymphoedema after breast cancer treatment can be devastating. It attacks confidence and self-esteem and can reduce a person's quality of life.
"Knowing that exercise can be beneficial will give confidence to those living with the condition. Exercise can also help maintain or improve health for anyone diagnosed with breast cancer.
"Regular movement every day can help keep joints supple and aid lymph drainage and extra exercises can also be useful if swelling restricts movement of the arm."