The Academy of Medical Royal Colleges has said it is "disappointed" at the prospect of further industrial action each month until the end of the year.
In a statement, it said: "We are acutely aware that the NHS is under extreme pressure at the moment. Patient safety and quality of care must be the priority.
"We know there are genuine concerns about the contract and working arrangements, but we do not consider the proposed strikes are proportionate.
"Five days of strike action, particularly at such short notice, will cause real problems for patients, the service and the profession."
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?