They found the risk of dying was almost a fifth higher in ICUs where more than 85% of beds were occupied, than in those running at between 45% and 85% capacity.

That meant a 60-year-old being treated in one of these units had the same risk of dying as a 70-year-old on a quieter ward. 

The Royal College of Emergency Medicine sets 85% as the maximum safe level of bed occupancy. 

However, the team found there was no tipping point after which deaths rose - instead, survival rates fell consistently as bed-occupancy increased.