Liam Byrne (Minister of State, Home Office) | Hansard source

Although Immigration Act powers of detention are not time limited, domestic and ECHR caselaw provides that detention must last for no longer than is reasonably necessary for the purpose for which it is authorised and must not be of excessive duration.

The detention of families with children is kept to the minimum period necessary and is subject to frequent and rigorous review, including ministerial authorisation in those cases where detention reaches 28 days. Where family detention is prolonged it is often because parents seek to frustrate the removal process. To introduce a time limit on detention would reward such behaviour and that would be unacceptable.