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."
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The 10K and half-marathon races take place almost exactly a year after Salman Abedi killed 22 in his attack at an Ariana Grande concert.
Dmitrios Pagourtzis also had several kinds of homemade explosive devices but they were not capable of detonating, county officials said.
After early mist clears, most parts of England and Wales will have another fine, warm day with plenty of sunshine.