GP surgeries in England have been ordered to stop half-day closures or face major funding cuts.
More than a quarter of a million GP appointments will be available as the NHS in England orders surgeries to end the practice of half-day closing.
More than 700 surgeries, or a tenth of England's total number, are currently thought to shut for part of the week, the NHS has said.
And the service said a small number of surgeries are opening for just 15 hours per week.
Half-day closures meant more than an estimated one million hours of GP time could have been lost last year, the NHS said.
As part of the NHS Long Term plan, practices will have to seek permission from local health authorities to shutter during working hours or risk losing funding worth more than £40,000 per business.
Dr Nikki Kanani, acting director of NHS primary care, said GPs will also be made to create more appointments before 8am and after 6.30pm.
She said: "By curbing half day closures we will be freeing up hundreds of thousands of appointments at a time when we know it can sometimes be tricky to see your GP.
"Whilst occasionally practices need to close for training that ensures the up to date skills of family doctors, our changes aim to further reduce regular half day closures and improve the availability of appointments across core hours in the week, supporting the best possible patient access in primary care."
