Women with babies and children were among 56 migrants packed into a small room in "shocking" conditions in Dover, the Home Secretary was warned.
Commons Home Affairs Select Committee chairwoman Yvette Cooper has written to Priti Patel raising "serious concerns" after MPs visited the Kent Intake Unit.
As well as issues over overcrowding and the length of stays, the committee was "very concerned" about the "clear risk" of a Covid-19 outbreak.
The visit came amid mounting concerns at the Government's handling of migrants arriving on British shores.
More than 9,000 people are estimated to have made the perilous crossing over the English Channel in small inflatable boats this year – outstripping the total for the whole of 2020.
MPs from the committee swooped on the unit this week.
“In September of last year, the Chief Inspector of Prisons warned that the Kent Intake conditions were unacceptable, particularly for child welfare,” former Shadow Home Secretary Ms Cooper told the BBC.
"To keep so many people in a small room like that for such a long time is clearly not appropriate.”
In her letter to the Home Secretary, Ms Cooper blasted the “shocking conditions the committee observed during its visit to the Kent Intake Unit”.
She wrote: "The holding room facility, in which detained asylum seekers wait for onward placement and screening, is wholly inappropriate.
“Yesterday there were 56 people packed into the small waiting room.
“The space is clearly unfit for holding this many people.
“Most people were sitting or lying on a thin mattress and those covered almost the entirety of the floor including the aisles between seats.
“Sharing these cramped conditions were many women with babies and very young children, alongside significant numbers of teenage and young adult men.
“We heard that the maximum period of time any individual should be held in this room is 24 hours but that in recent weeks some people have been kept in this small holding room for periods up to 36 and 48 hours."
The Home Office was approached for comment.
