In AC (Algeria) v Secretary of State for the Home Department  EWCA Civ 36, the Court of Appeal gave a trenchant warning that once it ceases to be lawful to detain an individual, the 'grace period' allowed within which to make arrangements for release can only be a short period. Moreover, the reasons for which any such grace period is required will be be closely scrutinised by the courts.
Unsurprisingly, there continue to be a very significant number of judicial review and county court claims for unlawful detention brought by current and former immigration detainees. What is perhaps more interesting is that despite the relatively well-understood law governing the lawfulness of immigration detention the precise legal limits of the Home Secretary's power to detain for immigration purposes continue to be tested and developed.
The lawfulness of the Home Secretary's exercise of the power to detain for immigration purposes is subject to the common law limits initially described in R v Governor of Durham Prison ex parte Hardial Singh  1 WLR 704 ('the Hardial Singh principles'), as clarified by Dyson LJ, in R (I) v SSHD  EWCA Civ 888 at [46-7] and approved in R (Lumba) v SSHD  UKSC 12 at [22, 24, 44, 103]:
- The starting point for any challenge to detention is that every imprisonment is prima facie unlawful and that it is for the Home Secretary to justify their act;
- The Home Secretary must intend to deport the person and can only use the power to detain for that purpose.
- The deportee may only be detained for a period that is reasonable in all the circumstances.
- If, before the expiry of the reasonable period, it becomes apparent that there is no realistic prospect that the Home Secretary will not be able to effect deportation within that reasonable period, the detention will be unlawful even if the reasonable period has not yet expired.
- The Home Secretary should act with the reasonable diligence and expedition to effect removal.
The 'grace period'
However, overlaid on top of the Hardial Singh principles had become the concept of a period of time during which continued detention is lawful in order to make suitable arrangements for release despite detention per se being no longer lawful when applying the Hardial Singh test set out above. The Home Secretary must act reasonably, but there would be 'allowance' made for the need for decision-making and to make arrangements. See for example R (AT- Guinea) v SSHD  EWHC 2709 (Admin) at .
The Decision in the High Court
The First-tier Tribunal had ordered that the Appellant be released from detention into immigration bail on 15 August 2018, by which point he had been detained for nearly 8 months. By the time of the hearing in the High Court in January 2019, he still had not been release apparently due to lack of suitable bail accommodation.
Jeremy Johnson QC (sitting as a Deputy High Court Judge) held in AC (Algeria)  EWHC 188 (Admin) having referred to the 'grace period' concept held that  that
the Hardial Singh principles are sufficiently flexible in their application to permit continued detention for the purpose of arranging appropriate bail conditions, once continued detention is no longer compatible with [Hardial Singh]. In other words, if it becomes apparent that it will not be possible to remove a person within a reasonable period of time, continued detention for a short period whilst arrangements are made for release on bail may be justified.
He went on to hold that while there was no real prospect of the Appellant being removed within a reasonable period of time, nevertheless his continued detention was lawful in order to arrange suitable accommodation.
The Court of Appeal's judgment
Irwin LJ referred to the phenomenon of the 'grace period' and held at  that the case
exemplifies an over-liberal approach to this problem. An increased energy and rigour should be required of the Secretary of State in relation to such final periods of detention.
Irwin LJ emphasised at  that once detention became unlawful applying the Hardial Singh principles then
The law recognises that does not mean the detainee must be ejected from detention, that day or the next, whatever the circumstances. But it does assuredly mean that the Secretary of State continues to detain on borrowed time, or in the language which has been adopted, is then and there enjoying a "period of grace".
He reviewed the relevant case law on grace periods and held at  that it showed:
 that the "grace periods" are granted for practical purposes, reflecting the facts of each case and applying a test of reasonableness;  that this court has declined to set any overall or absolute limit to such a period as a "long-stop" for all purposes;  that the periods have more usually been short, often a few days, but running up to a month, and  that there has been some tendency for the periods to increase.
In deciding for the Appellant, Irwin LJ concluded that once any of the Hardial Singh principles were breached, and detention became unlawful, then any further detention could be only be lawful for "for a reasonable period to put in place appropriate conditions for release." What was reasonable was fact specific, and did include the risk to the public, but the risk to the public could not justify indefinite immigration detention. On the facts of the case, he held that an appropriate grace period would have been five weeks.
This judgment will doubtless be an important aid for immigration detainees seeking to challenge their continued detention for no other reason than administrative delay. It will also put even more pressure on the Home Office and Probation Service to ensure that practical arrangements for effecting release start to be considered once release becomes a realistic possibility.