Almost 100,000 patients in England had to wait more than four hours on trolleys or chairs for a hospital bed after being assessed in A&E in December.
The number shot up by almost 10,000 in a month, with more than 2,000 people having to wait over 12 hours.
"This is something that is never supposed to happen in the NHS,” said RCN Chief Executive & General Secretary Dame Donna Kinnair. “By definition, these are the most seriously ill patients, and nurses fear patient safety is being compromised by having to care for people in corridors and on trolleys.
"In addition, last month, around 400,000 people had to wait more than four hours simply to be seen at A&E. This is shocking given that we haven’t yet suffered a really cold snap or severe outbreaks of flu or norovirus so far this winter.
"Hospitals' and care homes’ ability to treat patients more quickly and safely won’t improve until they can open more beds to cope with increasing demand – but that can’t happen without more nursing staff.
"The new government must begin work urgently to address the underlying causes of the current workforce crisis and recruit and retain more nurses, as committed to during the election campaign."