Accident and emergency units are facing a "perfect storm" that will heap on the pressure in the build-up to the busy winter period, according to one of the UK's most senior medics.
A lack of staff and possible industrial action by junior doctors were "very, very worrying" at this time of the year, Dr Mark Holland, the president of the Society for Acute Medicine, said.
He said there has been a rise in the number of patients needing emergency care at this stage of the year - and NHS England statistics due to be published today are expected to show a decrease in the number of patients being treated within four hours.
Last month's figures - which showed that 5.7% of patients were not treated on time - revealed the worst performance for August since monthly records began in 2010.
"There is already talk of putting ambulances on divert, of taking patients to other hospitals," Dr Holland told Sky News.
"That intensity is beginning to creep up. Not on a day-to-day consistent basis like last December and January, but it's starting and it feel like it is occurring a wee bit too early."
"The question this winter is how resilient we will be and what will be the tipping point. And that is the unknown factor which is very, very worrying."
He said hospitals have not had the traditional quieter summer period which enables them to catch up after the winter.
"Now in medical terms it feels like the summer no longer exists," he said. "It feels as if it is always the winter.
"You have got the perfect storm. You have got junior doctors in dispute with the Government, we can not recruit into all of our training posts and there are lots of consultant posts unfilled.
"Last winter we were pushed to the brink. All of these things coming together, it only needs one thing to tip over and you will get a domino effect."
Dr Holland said 30% of hospital beds are still taken by patients who do not need them, and promised to "spend every available penny" to fix the problem.
A spokesman for NHS England said: "Front line services are treating more patients than ever, and it's right to say that we need strong primary and social care to help offset the pressures on A&E.
"But it is also worth remembering that despite the usual flow of negative predictions at this time of year, our services continue to admit or treat and discharge more than nine out of 10 patients within four hours - a higher standard than any major western nation."