Was the arrest itself lawful?

Most of what this site covers assumes an arrest has already happened. This page is about the arrest itself — the power a constable is using when they arrest you without a warrant, and the conditions that power comes with.

Section 24 of the Police and Criminal Evidence Act 1984 (PACE) — wholly rewritten in 2006, so this is the current text, not the pre-2006 version some older guides still quote — sets out that power. It has two separate conditions, and both have to be met:

1. Reasonable grounds to suspect an offence. “If a constable has reasonable grounds for suspecting that an offence has been committed, he may arrest without a warrant anyone whom he has reasonable grounds to suspect of being guilty of it.”
2. Reasonable grounds to believe arrest is necessary. The power “is exercisable only if the constable has reasonable grounds for believing that for any of the reasons mentioned in subsection (5) it is necessary to arrest the person in question.”

Suspicion of an offence is not, on its own, enough. The power arises only where the officer also has reasonable grounds to believe arrest is necessary for one of the specific reasons Parliament listed — not for any reason the officer thinks is sensible at the time.

Section 24(5) sets out a closed list. The reasons include:

• To enable the person’s name to be ascertained, where it is not known and cannot readily be found out.
• Correspondingly, to ascertain their address.
• To prevent the person causing physical injury to themselves or anyone else, or damage to property.
• To protect a child or other vulnerable person from them.
• To allow the prompt and effective investigation of the offence or of their conduct.
• To prevent a prosecution being hindered by the person disappearing.

If none of these applies, the necessity condition is not met, whatever the strength of the suspicion.

Being arrested is not lawful just because the necessity condition is met on paper. Section 28 PACE adds a separate requirement: “the arrest is not lawful unless the person arrested is informed that he is under arrest as soon as is practicable after his arrest”, and separately, “no arrest is lawful unless the person arrested is informed of the ground for the arrest at the time of, or as soon as is practicable after, the arrest.”

This applies “regardless of whether” either the fact of the arrest or the ground for it looks obvious to the officer. An officer who assumes you already know why cannot use that assumption instead of telling you.

Under PACE Code G, “a person who is arrested… must be cautioned” unless it is impracticable because of their condition or behaviour at the time, or they were already cautioned immediately beforehand. The words are set:

“You do not have to say anything. But it may harm your defence if you do not mention when questioned something which you later rely on in Court. Anything you do say may be given in evidence.”

That caution is what gives shape to the right to silence at this exact moment — you are told, on arrest, both that you do not have to speak and what staying silent about something you later rely on can cost you.

Once arrested away from a police station, section 30 PACE requires that “the person must be taken by a constable to a police station as soon as practicable after the arrest.”

That duty is not absolute if the reason for the arrest falls away first. The Act requires release, without bail, if “at any time before the person arrested reaches a police station, a constable is satisfied that there are no grounds for keeping him under arrest”. This is sometimes called de-arrest: if the necessity that justified the arrest stops applying — the name is confirmed, the immediate risk passes — the power to keep holding you stops with it.

An arrest can be enforced with force, but the force itself has limits, not a blank cheque. Under s.117 PACE, an officer using a power under the Act “may use reasonable force, if necessary, in the exercise of the power”. Separately, under s.3 of the Criminal Law Act 1967, “a person may use such force as is reasonable in the circumstances… in effecting or assisting in the lawful arrest of offenders or suspected offenders”.

Both provisions describe the same scope: force is permitted only to the extent it is reasonable and necessary to carry out the arrest, not as a matter of degree left entirely to the officer’s discretion.

Comply now, challenge later. If you believe the two conditions in s.24 were not met, or that you were not told the ground for the arrest, that is not something to argue out at the roadside. Physically resisting an arrest — including one that later turns out to have been unlawful — risks a separate criminal offence of its own, on top of whatever is already being investigated.

The lawfulness of the arrest itself is a question for afterwards: through a formal complaint to the force, or, in a case where it matters, a civil claim. Note the time, what you were told and by whom, and raise your solicitor request as normal once you are at the station — a badly-grounded arrest is not made better by refusing to go along with it, and it is not made worse by going along with it either.