The Medical Defence Union (MDU) says practices are increasingly using text messages to communicate with patients.
Uses include reminding patients about appointments or to take medication, communicating test results or checking on a patient's progress.
But the MDU says it is unwise to rely on patients' implied consent to allow the practice to communicate with them in this way.
It urges GPs to adopt a cautious approach and seek patients' express consent.
Dr James Armstrong, MDU medico-legal adviser, said patients 'might be surprised to receive a text message from the surgery if they had not given their specific consent and this could lead to a complaint, particularly if the unexpected message is somehow picked up by someone else'.