Dr Sarah Jarvis has told Daybreak that it would be "difficult" to impose a seven-day rule on GPs and hospitals.
She said: "This is not going to be free, there's going to be a lot of bureaucracy and that means more cost for the NHS and patient care would suffer."
More top news
An Independence Day event at a UK base has been cancelled by the US Air Force over security fears.
British firms predict the economy should sustain a "solid" pace of growth for the rest of the year thanks to lower oil prices and inflation.
A charity has warned of a "pressure cooker environment" in schools putting children's mental health and self-esteem in danger.