Doctors are taking industrial action for the first time in 37 years today, despite last-ditch attempts from ministers to dissuade them.
Health Secretary Andrew Lansley pleaded with doctors not to take part in the day of action which is being held in protest over the Government's controversial pension reforms.
He urged doctors not to participate in the "pointless" strike, warning them it will achieve nothing.
NHS leaders also said patients should not be dragged into the dispute.
The British Medical Association (BMA) said its members did not want to participate in the day of action but feel that there is "no other option left" to make their voices heard.
The BMA announced the action last month after it accused ministers of pressing ahead with "totally unjustified" increases to pension contributions and a later retirement age fordoctors even though a deal on pensions was agreed four years ago.
The union announced the move after a ballot of more than 100,000 doctors showed a clear majority in favour of protests on a high turnout of 50%.
All non-urgent work will be postponed, the BMA said, adding that although the action will be disruptive, doctors will ensure patient safety is protected.
Doctors will see anyone who is ill, or who believes they are ill, on the day of action but will not do paperwork.
Accident and emergency departments and maternity services will run as normal, and tests for critical conditions such as cancer will be still available.
But some non-emergency hospital clinics, outpatient appointments and planned surgery may be postponed.
GP practices also will remain open but some may postpone routine appointments which can be rearranged for another day.
According to Mr Lansley, the planned action could see up to 30,000 operations cancelled, 58,000 diagnostic tests postponed and 200,000 outpatient appointments rescheduled.
Up to 1.25 million GP appointments will be pushed into the days and weeks following the action, he said.
Local NHS managers have urged patients to use services only if there is an urgent need.
The BMA has more than 100,000 members, all of whom could take part in the action. However, early polls suggest as few as 22% of doctors will be participating.
The last time doctors took action was in 1975, when consultants suspended goodwill activities and worked to contract over a contractual dispute, and junior doctors worked to a 40-hour week because of dissatisfaction with the progress of contract negotiations.