Patients at the Queen Elizabeth University Hospital who arrive the previous day are being left over 12-hours or more until there is space to move them to the appropriate wards.
Sources told the Evening Times that there have been difficulties recruting specilist staff for the ward, with workers from other departments covering its needs.
A source said: “Most days start with significant numbers of patients from the previous day still waiting for a bed.”
The capacity of the unit has already been expanded since the multi-million pound hospital opened last May.
Extra cubicles were announced by the Scottish Government before Christmas after an elderly patient died on a trolley in the department sparking a review.
Paisley GP Dr John Ip said the unit had reached “crisis point”.
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He said: “People were waiting hours and hours on trolleys. These are frail, elderly people.
“This hospital is going to be there for 50 to 60 years at least. We need to make it work. We do not have the funds to build a new hospital.”
New figures, obtained by The Herald, have revealed that during the first five months of opening almost 3000 patients were kept in the unit for more than 12 hours.
There are no set waiting time targets to be met in Scottish hospital units, unlike the four-hour waiting time for accident and emergency departments.
But Dr Ip has said that it is common for patients waiting on trolleys and that they could be deteriorating or in pain.
NHS GGC released a statement saying: “The immediate assessment units are not emergency departments.
“They are part of a hospital admissions unit where patients come in for a series of tests and assessments to determine if they need to be admitted for a longer stay in hospital or whether they can go home with appropriate support.
“Patients can therefore be expected to stay for longer than 12 hours in the unit.”