The Government is living in "cloud cuckoo land" over seven-day working, a Royal College has said, as it published figures suggesting almost a million more people will be living with long-term conditions by 2025.
The Royal College of GPs (RCGP) predicts an "'explosion" of people living with more than one serious long-term, life-threatening condition in the next decade.
It said the number will rise from an estimated 8.2 million in 2015 to 9.1 million in 2025.
This will cost the NHS up to £1.2 billion a year, it said.
Addressing the RCGP conference in Glasgow, chair Dr Maureen Baker will say the Government's plans for seven-day working is a "recipe for disaster".
Addressing Health Secretary Jeremy Hunt, she will say: "Mr Hunt you say 'new deal', but my message to you and Mr Cameron is this: If you don't shore up existing GP care as your top priority, not only will you not get a seven-day service, but you won't have a five-day service either - because you will have completely decimated general practice."
She will call for an immediate increase in funding to ensure general practice receives 11% of the overall NHS budget, including "an immediate injection" of £750 million.
General practice in England receives just 8.45% of the NHS budget, she will say.
Dr Baker will say every stop must be pulled out to increase the number of GPs, including financial incentives to attract medical students to general practice.
She will argue that delivering care to patients with chronic conditions is most cost-effective in general practice, yet most NHS money goes to hospitals.
Dr Baker will talk about growing up in a working class family in Wishaw in Lanarkshire, Scotland.
Her father, one of 10 children, died in his 50s, and the lives of her uncles and aunts were shortened by heart disease, cancer and respiratory disease.
Nowadays, their life expectancy would most likely be significantly longer, she will say.
"They might be living into their 70s, 80s, and beyond, and would almost certainly be coping with one, two, three or more long-term conditions.
"It is a great testament to modern medicine that nowadays we are much more likely to prevent or treat diseases that in the past killed people so early in their lives.
"GPs have played a pivotal role in this transformation.
"But this success has brought with it a whole new set of challenges to which the NHS is currently struggling to respond."
According to the RCGP, GPs and practice teams make 370 million patient consultations a year - 60 million more than just five years ago.