The government's 10-year plan for the NHS lacks both the staffing and funding to succeed, Labour has said.
The PM has promised the publication of the plan on Monday will lead to "world class" care for patients in England.
Pledges on maternity care, mental health, elderly support and earlier detection and prevention of diseases will be included in the plan.
But shadow health secretary Jonathan Ashworth said Mrs May was just trying to "clear up a mess that she has made".
Appearing on the BBC's Andrew Marr Show, Mr Ashworth said: "The funding isn't sufficient and the staffing isn't there."
He added the NHS "doesn't need 10 more years of the Tories, it needs a Labour government".
However, Mrs May said that, coupled with the extra money announced last summer, the plan secured the future of the NHS.
The budget is due to rise by £20bn a year above inflation by 2023, though a detailed explanation of where that money will come from has not yet been provided.
Health Secretary Matt Hancock, meanwhile, admitted the health service was short staffed but said work was being done to solve the problem.
He told BBC News there were "still lots of people" trying to become nurses in the UK and that there was a "record number of GPs in training".
If you can't see the NHS Tracker, click or tap here.
A day before publication of the 10-year plan, the prime minister said the proposals marked "a historic step" to secure the future of the NHS, adding there was a focus on "ensuring that every pound is spent in a way that will most benefit patients".
"This will help relieve pressure on the NHS while providing the basis to transform care with world-class treatments," she said.
When asked on the Andrew Marr Show why the NHS was failing to hit key waiting time targets, Mrs May blamed an increased in demand on health services.
The 10-year plan, she added, was "crucial" for ensuring the "sustainability" of the NHS.
Speaking to the BBC in June, Mrs May suggested funding for the plan would come from a combination of taxes and the so-called "Brexit dividend".
However, during the summer, Commons Health and Social Care Committee chairwoman Sarah Wollaston described the idea of a Brexit dividend as "tosh".
As healthcare policy is politically devolved, the plan only applies to the NHS in England but Mrs May said the other UK nations will be drawing up their own plans. Under the government's funding system they are getting an extra £4bn between them by 2023.
What's in the plan?
The full details are not being unveiled until Monday when it will be released by NHS England.
But Mrs May has outlined some of the key focuses. These include:
- Better mental health care, including round-the-clock advice from NHS 111 by 2023 and tailored services for young adults. Currently once someone in care turns 18 they are thrust into the adult system, often when they are not ready
- Providing the best maternity care in the world by improving safety and providing greater mental health support for new parents. One in five new mothers struggles with mental health in the first year of her baby's life
- Greater control and choice in old age by expanding the use of personal budgets to allow people to decide what care they want, and greater support in the community so people do not end up in hospital
- Better prevention and detection of disease - cancer is expected to be a key focus with an ambition to increase the number of early detections from one in two cancers to three in four, which in turn will improve survival
- Increases in the NHS workforce - currently one in 11 posts is vacant
- Bringing the NHS into the digital age, including online GP booking, prescriptions management and health records
Why is it being published now?
The plan was promised during the summer when the government unveiled its funding settlement for the NHS.
That set out the budget for the next five years and means by 2023 funding for the NHS will be £20bn a year more than it is now, once inflation is taken into account. That is the equivalent of annual "real terms" rises of close to 3.5% - about twice what the NHS has got since 2010.
At the time, the prime minister said she wanted to ensure the money was used wisely and so asked NHS England boss Simon Stevens to draw up a long-term plan for the next decade.
It was expected to be published in the autumn, but was delayed because of the government's troubles getting its Brexit plans agreed.
The last time such a long-term vision was set out was in 2000 under Tony Blair.
How are people reacting?
Understandably people want to see the full details before coming to firm conclusions. But the priority areas are being welcomed.
Andy Bell, of the Centre for Mental Health, said the initiatives on mental health were much needed.
He said: "For too many young people, mental health support is offered too late, with too many restrictions and then they are forced to start again when they reach 18."
Dame Donna Kinnair, of the Royal College of Nursing, said nurses shared the ambitions being set out.
But she added the government needed to "urgently address" the staffing shortages if it was going to succeed.
And Jennifer Dixon, chief executive of the Health Foundation think-tank, predicted fulfilling the pledges would be "extremely tough" because of the scale of staffing shortages, rising pressures and cuts to other parts of the wider health and care system.
The £20bn promise just relates to the front-line NHS budget and so does not cover other elements such as social care and public health - at the end of last year it was announced the budget for these services, which includes smoking cessation and weight management, was being cut by over 4% in real terms next year.
Ms Dixon said: "Trade-offs are inevitable and these must be spelled out clearly so the public know what they can expect from the NHS."
What are we not being told?
There has been a fair bit of tension behind the scenes. The Treasury is understood to have wanted to tie the NHS down in terms of what it will achieve.
One of the central bones of contention is thought to be how the NHS can tackle deficits and waiting times.
Hospitals are struggling to balance their books and have seen a deterioration in the time patients wait in A&E, for cancer and for routine operations.
None of these three key waiting time targets are currently being met.
It looks like a trajectory for improvement will be published at a later date with NHS bosses known to have been wary about promising things they felt they could not deliver.
Another missing piece is the green paper on social care.
This was first promised in 2017, but has been delayed on a number of occasions. Brexit has certainly been a factor, but again there has been disagreements in private, this time over how radical the plan should be given the problems facing the sector, which covers care homes and home help.
The government has promised the green paper will be published as soon as possible.