The junior doctor strike represents a "serious escalation" of the ongoing dispute and is a "great concern", according to the head of the General Medical Council (GMC).
Niall Dickson, chief executive of GMC, said: "It is obviously a matter of great concern for everyone, especially for patients, and when so little time has been given for the NHS to make contingency plans.
"We recognise the frustration and alienation of doctors in training and indeed their right to take industrial action. However, we issued advice earlier in this dispute both to senior doctors and doctors in training, and we will now consider whether further guidance is needed."
He added: "The first priority must be to protect patients from harm.".
The medical regulator has urged junior doctors to seriously consider whether or not to take part in a new wave of strikes.
A five-day strike by junior doctors has been called in the ongoing dispute over contracts, but what does it mean for patients?