But some people were held for far longer periods due to a lack of alternative accommodation.
The BBC reports the emails show Home Office permanent secretaries were aware of overcrowding concerns at the time.
“Their detention is no longer legal as they can only be detained whilst their identity is locked down and then only for a maximum of 5 days,” one email said.
“Most have been there for a number of weeks, longer than some Manston cases. We need to move them to hotels ASAP…”
A Home Office spokesperson told the broadcaster an unprecedented number of small boat arrivals had put “huge pressure” on the asylum system, adding it had “worked tirelessly to move people into hotels or other accommodation as quickly as possible”.