Sir Keir Starmer has urged resident doctors not to go ahead with "reckless" strikes planned for next week, saying it is "beyond belief" they could take place during a flu outbreak.
The prime minister said the NHS is in its "most precarious moment" since the coronavirus pandemic due to soaring flu cases across the UK, and that strikes would put the healthcare system and its patients in "grave danger".
The doctors' union, the British Medical Association (BMA), is polling its members to see if they are willing to call off the walkout, with the results to be released on Monday.
But if they vote against it, a five-day strike by resident doctors will begin two days later on Wednesday 17 December.
A BMA spokesperson said the government could stop the strike by tackling pay, conditions and trust with a credible offer.
