The number of people in England’s hospitals with flu quadrupled in the last month, according to NHS data.
A spokesperson for the hospital said it had a “comprehensive plan in place” and was “taking all the necessary actions to manage the challenging circumstances”.
“We are working with partner organisations to ensure those that are medically fit can leave hospital safely and at the earliest opportunity,” they added.
People whose case is not an emergency are being asked to see their GP, pharmacy or walk-in centre – or call the 111 service for advice.
The hospital warned some people would experience delays as it prioritises the sickest patients.
The Royal Liverpool University Hospital is in the city centre and is the biggest hospital in Merseyside and Cheshire.
Declaring a critical incident can happen when a hospital is experiencing exceptional demand, or sometimes if there is a serious problem with staffing levels.
It indicates it cannot function as normal and allows it to take extra measures to protect patients, such as prioritising the most unwell people and getting support from other agencies.
It could last hours, a few days, or weeks if necessary.
A critical incident was also declared on Friday by the NHS Cornwall and Isles of Scilly Integrated Care Board.
It said it had seen almost four times as many inpatients compared with last year and urged people with flu to avoid going to A&E.
