The A47 has re-opened between Wisbech and Guyhirn after a man suffered a cardiac arrest after being stung by a wasp.
Emergency services, including police, ambulance, and the Magpas Air Ambulance were called to the scene.
A Cambridgeshire police spokesman said: “We were called today by the ambulance service at 12.54pm (Wednesday) with reports of a man having been stung by a wasp and going into cardiac arrest on the A47, approximately one mile from the Wisbech roundabout, heading towards Guyhirn.”
An East of England Ambulance Service spokesman said: “We were called to Wisbech to reports of a man who had collapsed and was unconscious.
“When we arrived we found a male, believed to be in his 50s, who was in a serious condition.
“He was taken to Queen Elizabeth Hospital in King’s Lynn, where he was in a critical condition.”
The road was re-opened at about 2pm.