Health officials have raised concerns over the safety of patients after junior doctors announced plans to carry out the first full walkout of doctors in the history of the NHS.
The move, announced by the British Medical Association (BMA), comes as a result of the dispute with the Government over new working conditions.
In the second of two strike actions planned for April in England, junior doctors - everyone up to consultant level - will do a full walkout, the BMA said. In previous days of industrial action doctors have still provided emergency cover.
The BMA said the move follows the "continued refusal by the Government to step back from its decision to impose a new contract on junior doctors from August this year and resolve the dispute by re-entering talks".
The Department of Health said a full walkout would "inevitably put patients in harm's way".
And NHS England said the move will "undeniably put the health services' sickest patients at greater risk".
The doctors' union has two sets of action planned. A 48-hour strike action starting on Wednesday April 6 at 8am, where junior doctors provide "emergency care only", will go ahead as scheduled, the BMA said.
But the industrial action set for April 26 and April 27 will now be a "full withdrawal of labour" by junior doctors between the hours of 8am and 5pm on the two days.
Dr Johann Malawana, chairman of the BMA's junior doctor committee, said: "No junior doctor wants to take this action but the Government has left us with no choice. In refusing to lift imposition and listen to junior doctors' outstanding concerns, the Government will bear direct responsibility for the first full walkout of doctors in this country.
"The Government is refusing to get back around the table and is ploughing ahead with plans to impose a contract junior doctors have no confidence in and have roundly rejected.
"We deeply regret the disruption to patients and our message to patients is clear; this action is wholly avoidable but the Government must choose talks over imposition.
"For the sake of patients, doctors and the future of the NHS the Government must put politics to one side, get back around the table and end this dispute through talks."
An NHS England spokesman said: "The proposed withdrawal of emergency cover is a deeply regrettable development that undeniably would put the health services' sickest patients at greater risk.
"The rest of the NHS will be pulling together to the greatest possible extent, but no-one should try and pretend this will not place real pressure on already hard-pressed urgent and emergency care services."
A Department of Health spokeswoman said: "This escalation of industrial action by the BMA is both desperate and irresponsible - and will inevitably put patients in harm's way.
"If the BMA had agreed to negotiate on Saturday pay, as they promised to do through Acas in November, we'd have a negotiated agreement by now.
"Instead, we had no choice but to proceed with proposals recommended and supported by NHS leaders."
Junior doctors have carried out three strikes so far in this dispute, but on each occasion they have still staffed emergency care.
Talks between the Government and BMA broke down in January - the major sticking point has been over weekend pay and whether Saturdays should attract extra "unsocial" payments.
After talks broke down the Government announced in February it would be imposing the contract from August.
Jeremy Taylor, chief executive of National Voices, a coalition of 160 health and care charities, said: "The Government and the BMA seem unable to get around the negotiating table, and the only people who will suffer are patients.
"We are calling on government to drop the imposition of a new contract, the BMA to call off planned strikes, and both sides to get back around the negotiating table as quickly as possible."