Thousands of junior doctors will go on strike today despite last-ditch efforts by David Cameron to get them to abandon their "damaging" course of action.
Across England, around 4,000 operations and procedures have been cancelled, with thousands more routine appointments also expected to be postponed.
Patients have been told hospitals are under pressure and asked to attend A&E only if they have a genuine emergency.
Instead, patients are being asked to make the most of other NHS services, including GPs, walk-in centres, the 111 phone line and pharmacies.
Around 100 picket lines have been put in place for Tuesday's strike, with a large concentration in and around London.
Yesterday, the Prime Minister pleaded with doctors to call the strike off and urged medics "at this late stage" to get back around the table.
He said: "This strike is not necessary, it will be damaging.
"We are doing everything we can to mitigate its effects but you can't have a strike on this scale in our NHS without there being some real difficulties for patients and potentially worse."
NHS England said 1,425 inpatient operations and procedures were being cancelled as a result of the strike along with 2,535 outpatient ones.
It said there are around 4,000 cancellations in total, of which 3,400 are on Tuesday.
Some 654 cancellations - 192 inpatients and 462 day cases - are in London.
Talks aimed at resolving the dispute over a new contract failed on Friday, although further talks will continue.
Junior doctors are set to provide emergency care only for 24 hours from 8am this morning.
This will be followed by a 48-hour stoppage and the provision of emergency care only from 8am on January 26.
On February 10, there will be a full withdrawal of labour from 8am to 5pm.
The basis for the current round of negotiations is the Government's offer from early November, including an 11% rise in basic pay for junior doctors.
This is offset by plans to cut the number of hours on a weekend for which junior doctors can claim extra pay for unsocial hours.
Currently, 7pm to 7am Monday to Friday and the whole of Saturday and Sunday attract a premium rate of pay.
Under the Government's offer, junior doctors would receive time-and-a-half for any hours worked Monday to Sunday between 10pm and 7am, and time-and-a-third for any hours worked between 7pm and 10pm on Saturdays and 7am and 10pm on Sundays.
The BMA has said there are still several areas of dispute, despite Mr Hunt saying the only sticking point is weekend pay.
In an interview over the weekend, Mr Hunt said the Government was going through the "exhaustive process" of contacting every A&E department in the country to establish whether they would have enough staff to stay open.
But he admitted "hospitals are stretched at the moment".
Unite head of health Barrie Brown said: "Unite's 100,000 members in the health service will be giving the doctors maximum support within the bounds of the law by joining protests outside their working hours and taking to social media to highlight their support.
"The fact that this is the first industrial action by doctors since 1975 demonstrates the cack-handed and ideological way that ministers have dealt with the NHS since 2010 - the chickens have come home to roost."
RMT general secretary Mick Cash said: "RMT members are urged to join the picket lines at their local hospital tomorrow to show solidarity with the junior doctors at this crucial point in their fight for justice."