Operations will be cancelled or delayed, while casualty and intensive-care departments could be severely undermanned.
Many patients who have been waiting months for routine surgery - such as a hip replacement or hernia repairs - will face the misery of having to rejoin waiting lists all over again.
Doctors and MPs raised fears that patient safety could be put at risk as the hospitals struggle to cope with staff shortages caused by the Government's botched on-line recruitment system.
An unprecedented 30,000 junior doctors will change jobs on Wednesday
Remedy UK, which represents junior doctors, bluntly warned patients: 'Wednesday is a bad day to go into hospital.' It warned that the knock-on effects will last for weeks.
Junior doctors change jobs every six months to gain experience in different areas of medicine. Even on day one of their new job, they play an essential part in providing health care to patients.
The job transfers used to be staggered throughout the year. But as part of changes to the junior doctor system, 30,000 will start work or change posts on the same day - tomorrow.
Many will have no experience in the areas of medicine in which they are working, will not know their way around the hospital and will be absent from the wards as they attend induction days.
Many hospitals will be overstretched as around 1,000 posts have yet to be filled as a result of delays in processing candidates under the shambolic Medical Training Application Service.
Dr Matt Jameson Evans, from Remedy UK, added that some doctors are being forced to work up to 25 days with only one day off. 'They are deeply worried about how they are going to cope with the exhaustion.'
Ministers were forced to abandon the new recruitment system earlier this year after on-line security lapses. The failures have led to a frantic rush to fill the remaining posts.
Despite this, 16,000 junior doctors have not found new posts and many have been forced to find work abroad.
Dr Jameson Evans said: 'We don't want to alarm patients, but we also don't want to cover up the Government's gaffes.
'We are being contacted by a large number of senior doctors who are very concerned about patient care on August 1 - including those working in intensive care and accident and emergency.
'What is happening is that the doctors who are in place are being forced to cover the shortages. They are going to have to work illegal hours and will be absolutely knackered.
'An already-overstretched service will be pushed to breaking point and that will undoubtedly increase the clinical risk to patients.'
A recent survey of 100 hospital trusts found that nine in ten have already cancelled some operations, while nearly one in five have had to take on expensive locums and temporary staff at extra cost to the taxpayer.
The frantic rush to fill vacant places also means that many junior doctors - including those recruited from abroad - will not be properly security checked before starting work. The Tories have described the lapse as 'disastrous' in the wake of controversy surrounding the alleged attempted car bombings in London and Glasgow by NHS workers.Michael Summers of the Patients' Association said: 'It is extraordinary`