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
Plumbers and heating engineers should be trained to help people living in homes that are too cold, according to new health guidelines.
Rain for the northwest, dry and cold elsewhere
Rejecting a showdown with Ed Miliband, David Cameron's terms sent to broadcasters reveal his deep dissatisfaction with the debate proposals.