text: Dame Sally Davies said the action by the British Medical Association (BMA) would "lead to patients suffering".
  The strike begins across England at 08:00 GMT, from when junior doctors will only provide emergency care.
  The BMA said the strikes "demonstrated the strength of feeling amongst the profession".
  It announced three spells of strike action in England in November, after negotiations with the government ended without resolution.
  Issues being disputed by the BMA and NHS include weekend pay.
  "As a doctor, I can understand the anger and frustration felt by many junior doctors at this time," Dame Sally said.
  "In part, this dispute is a symptom of frustration and low morale that has been building for decades and the strain that a career in medicine can place on your work-life balance.
  "Junior doctors are the backbone of the NHS, working long and anti-social hours... It is vital that, as senior medical leaders, we ask ourselves whether we are doing everything we can to ensure our junior colleagues feel valued."
  The planned strikes are set to take place from:
  Conciliation service Acas has confirmed that talks between the BMA and NHS bosses will continue next week.
  Junior doctors' leaders are objecting to the prospect of a new contract.
  The government has described the current arrangements as "outdated" and "unfair", pointing out they were introduced in the 1990s.
  Ministers drew up plans to change the contract in 2012, but talks broke down last year.
  The government has indicated it will impose the new contract next year in England. The BMA has responded by initiating the industrial action process.
  Junior doctors row: What you need to know
  What exactly do junior doctors do?
target: Junior doctors should suspend Tuesday's strike action over pay and conditions while talks continue, the chief medical officer for England has urged.