Doctors taking industrial action on Thursday could still be paid, it has emerged.
Health trusts have to decide whether or not to dock the pay of doctors who will only treat those in urgent need.
Up to 100,000 doctors who are members of the British Medical Association (BMA) could take part in the day of action in protest over the Government's controversial pension reforms.
Andrew Lansley: 'Up to 30,000 operations could be cancelled'
Meanwhile, hospital doctors who are not participating in industrial action on Thursday will be drafted in to other departments to ensure the safety of patients.
Staff who are willing to work will be expected to cover frontline services "if clinically appropriate", according to NHS London.
Local NHS managers have also reached out to patients to urge them to only use services if there is an urgent need.
Non-urgent care in hospitals and routine appointments with GPs will be affected by the action, but emergency care will not be affected.
NHS South Essex asked patients not to contact their GP surgery or attend accident and emergency wards unless there is an emergency or an urgent care need.
It also advised people to contact pharmacists for information and advice on minor illnesses and ailments.
Health Secretary Andrew Lansley said that, as a result of the planned action, up to 30,000 operations could be cancelled, 58,000 diagnostic tests may be postponed and 200,000 outpatient appointments may have to be rescheduled.
Lansley said up to 1.25 million GP appointments would have to be pushed into the days and weeks following the action.
The BMA announced the day of action last month after it accused ministers of pressing ahead with "totally unjustified" increases to pension contributions and a later retirement age for doctors even though a deal on pensions was agreed four years ago.
The BMA said all non-urgent work will be postponed.
The union said 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.
Most doctors will be taking industrial action for the first time, with the last dispute almost 40 years ago.
Meanwhile, the Royal College of Midwives has advised its members not to do anything to "undermine" the industrial action.
It urged members to "work as normal" on Thursday.
Jon Skewes, director of employment relations and development at the RCM, said: "This is legitimate action by our BMA medical colleagues and we respect their decision. We are advising our members to work as normal but not to undertake work that would undermine the BMA action.
"Our members voted to accept the Government's pension offer and with reluctance the RCM Board agreed. It is the best deal that could be obtained by negotiation. We will therefore be engaging with other NHS unions on the implementation of the pensions offer accepted by RCM members.
"Midwives have a profound sense of duty to the women and babies they look after, and they will continue to deliver the best care possible on the day."