NHS Direct is handling more than 4,000 calls a week from people suffering from suspected norovirus.
The number seeking help for diarrhoea and vomiting has been rising in recent weeks as the virus begins to take hold throughout the UK.
In November, NHS Direct received a total of 17,700 calls about these symptoms, which are most often caused by norovirus. It is the highest number of calls per month since March and is in line with expectations for the time of year.
The Health Protection Agency (HPA) has analysed the data for NHS Direct's call volume and found it is consistently high enough to suggest the "beginning and continuation of periods of norovirus activity nationally".
Separate data from the HPA shows there have been 91 suspected outbreaks of norovirus in hospitals since the summer, 74 of which have led to ward closures.
The HPA said the number of reports from hospitals is rising but is in line with expectations.
Most people with the bug will make a full recovery within two to three days and are advised to stay at home, take paracetamol to relieve fever and drink plenty of water to replace lost fluids.
Drinking fluids is particularly important for vulnerable groups such as young children and the elderly.