Maternity units, intensive care units and emergency departments will be at "most risk" when junior doctors stage their all-out strike next week, the Health Secretary has said.
Jeremy Hunt said he hoped the British Medical Association (BMA) would work with NHS England to fill rota gaps despite the strike.
He said that the full walk-out is a bridge the NHS has "never crossed before" but added that "no-one wants there to be any kind of tragedy".
Junior doctors are expected to stage a fifth strike on April 26 and 27 when they will fully withdraw labour between the hours of 8am and 5pm.
In previous strikes junior doctors provided emergency care cover but the industrial action planned for next week will see junior doctors stage a full walk-out - the first in the history of the NHS.
Speaking in the House of Commons, Mr Hunt said: "The departments at most risk are emergency departments, maternity departments and intensive care units and those are the areas which we are most keen to make sure that we maintain critical doctor cover over the two strike days that are planned.
"I really do hope that the BMA will cooperate with NHS England as we identify where we think gaps might be.
"We will share those with the BMA and I hope very much they will help us to plug those gaps with junior doctors because in the end no-one wants there to be any kind of tragedy."
A BMA spokeswoman said: "No junior doctor wants to have to take any action, they would rather be in hospital caring for patients, but they have already done everything else in their power to make their voices heard. By continuing to ignore them, the Government has left them with no alternative.
"The critical message for patients is that anyone who needs emergency care on the days of industrial action will get it, the only difference is that it will be provided by senior doctors rather than junior doctors. Furthermore, the BMA notified Trusts several weeks in advance, giving them time to plan ahead.
"For the sake of patients as well as doctors, the Government must listen to concerns from all sides calling on it to lift imposition and get back around the negotiating table. It is not too late to remove the threat of imposition and end this dispute through talks."
Speaking in the Commons, Mr Hunt faced repeated calls to return to the negotiating table with doctors' leaders.
But he said that the negotiations had been going on for three years and were "clearly not going anywhere at all".
He also insisted the Government does intend to "impose" the new contract.
There has been speculation that the Health Secretary may have performed a U-turn on how the new contract will be implemented amid suggestions that he may not have the legal powers to actually force it upon junior doctors.
But Mr Hunt rubbished the suggestion as he responded to an urgent question from Labour on the issue.
He said: "This House has been updated regularly on all developments relating to the junior doctors' contract, although there has been no change whatsoever in the Government's position since my statement to the House in February."
He said the deal will begin to be implemented in August as part of a phased approach.
"This Government has a mandate from the electorate to introduce a seven-day NHS and there will be no retreat from reforms that save lives and improve patient care," he said.
"Modern contracts for trainee doctors are an essential part of that programme and it is a matter of great regret that obstructive behaviour from the BMA has made it impossible to achieve that through a negotiated outcome."