The Prime Minister has "unreservedly condemned" a strike which ambulance workers are threatening to stage next week.
David Cameron said it would be "completely wrong" for the action to go ahead, particularly at a time of increased concern.
He told MPs during Prime Minister's Questions: " I hope there is something which all sides of the House can unite over - that it would be completely wrong for the ambulance strike that is proposed to go ahead next week.
"I unreservedly condemn any attempt to go on strike and to threaten our services, particularly at this time of heightened national concern.
"I hope members of the Labour Party - irrespective of which union they are sponsored by - will do the same thing."
He was responding to Labour's Angela Smith (Penistone and Stocksbridge) who asked what the Government planned to do to avert the "nationwide crisis" in the NHS.
The Tory leader added: "What we are doing in terms of the ambulance service is making sure there are the 17,000 extra paramedics.
"We have put £50 million more into the ambulance service over the winter."
The industrial action, planned by Unison, Unite and the GMB, means ambulance crews will strike from 12pm until midnight on Thursday of next week. Thousands of other NHS workers are also due to walk out from 9am to 9pm on the same day.
Mr Cameron's criticism echoes that of Health Secretary Jeremy Hunt who attacked the unions and said everything will be done to keep vulnerable patients safe during industrial action.
He has has written to unions repeating the Government's view that the cost of meeting a recommended 1% rise for all NHS staff would pay for thousands of extra nurses.
New talks are to be held today in a last-ditch bid to avert the planned walkouts in England and Northern Ireland on January 29.
NHS England has warned that the strikes raise a "real risk" that patients with life-threatening conditions are harmed.