Junior doctors are to take part in three more strikes and launch a judicial review as part of their fight against the government's decision to impose a new contract in England.
The government condemned the announcement by the British Medical Association, which comes two weeks after ministers said they would force the changes on doctors from the summer.
Ministers say the contract will improve services, and strikes are unnecessary.
The BMA says the contract is unfair.
The latest move marks a further escalation in the long-running dispute, which is fast becoming the most bitter breakdown in relations between the government and medical profession since the NHS was created.
The three strikes in March and April will each last 48 hours, although emergency cover will be provided.
The stoppages will be held on:
- Wednesday 9 March from 8am
- Wednesday 6 April from 8am
- Tuesday 26 April from 8am
Two strikes have already been held this year - one in January and one in February. Both lasted just 24 hours.
BMA junior doctor leader Dr Johann Malawana said he had spent the last two weeks consulting with members and they had given a "resounding message" that they "cannot and will not accept" the imposition, which includes paying doctors less to work Saturdays.
"The government must put patients before politics, get back around the table and find a negotiated solution to this dispute," he added.
The BMA also believes it has found legal grounds for a challenge.
Under the Equality Act 2010, there is an onus to show "due regard" to equalities issues, typically through carrying out an equality impact assessment.
This would look at issues such as whether access to services could be compromised by a change in policy.
The BMA said the government has failed to provide evidence such an assessment has been done and so will challenge the government in the courts over it.
A Department of Health spokesman said: "Further strike action is completely unnecessary and will mean tens of thousands more patients face cancelled operations - over a contract that was 90% agreed with the BMA.
"We urge junior doctors to look at the detail of the contract and the clear benefits it brings.
How far apart were the two sides?
- The BMA wants everyone who works on a Saturday to be paid at 50% above the basic rate
- The government is only offering extra pay after 5pm
- But they have offered to top up the pay by 30% for those who work regular Saturdays - defined as at least one in four
- Agreement has not been reached on on-call allowances, how limits on working hours are to be policed and days off between night shifts
- The government has offered a basic pay rise of 13.5%
- The BMA has said it is willing to accept between a 4% and 7% hike in basic pay to cover the weekend pay issue
The news comes hours after the government's former patient safety adviser said ministers were wrong to impose the contract.
Prof Don Berwick, who led a review for government on safety three years ago, said there should be a three-year moratorium on the imposition of the new contract and called on the government to "apologise".
He said the NHS had a "demoralised" workforce and needed to find another way out of the "mess", the Health Service Journal reported.
Speaking at an event in London at the King's Fund think-tank, Prof Berwick, who has also acted as a health adviser to US President Barack Obama, said: "You cannot achieve excellence in combat with your future workforce, it makes no sense at all."
The Prime Minister's spokeswoman said the freshly announced strikes were "regrettable", but she was sure discussions would continue.
And Danny Mortimer, chief executive of NHS Employers, which has been negotiating with doctors on behalf of the government, added the move was "disappointing" as the majority of the BMA's concerns had been addressed in the final contract offer.
"This disruption to patient care is unnecessary. I strongly believe that the final contract is safe, fair and reasonable. For the sake of the NHS, and patients I urge all junior doctors to take a look at the contract in detail before taking part in any future action."