The stand-off centres around lack of higher payments for working on weekends, referred to as anti-social hours.
The strike will also involve doctors withdrawing emergency services for the first time since the row began last year, with mass resignations and a high court challenge also in the works.
Junior doctors are planning two one-day strikes tomorrow and on Wednesday, walking out between 08:00 AM and 17:00 PM (local time) each day.
"It is not true that emergency care is being withdrawn on Tuesday and Wednesday. It is true that junior doctors won't be providing it, but hospitals across England will be full of senior doctors who will be delivering that care," British Medical Association (BMA) Chairman Mark Porter said.
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Junior doctors, a term covering nearly 50,000 medical professionals in the UK who are fresh out of medical school as well as those with more experience behind them, have warned that the new contract will affect patient safety by encouraging unsafe shift patterns and also financially hit doctors who work the longest hours.
"The government has left junior doctors no alternative other than what's happening this week. We have said repeatedly and always said that we will call off the strike if the government will call off the imposition (of the new contract).
"By contrast, the government has said, over the weekend, that there is nothing that will get it to call off the imposition," Porter said.
UK health secretary Jeremy Hunt has argued that he wants to improve care on Saturdays and Sundays, saying research shows patients are more likely to die if they are admitted during a weekend.