As per S24 PACE if a constable has reasonable grounds for suspecting that an offence has been committed, they may arrest without a warrant anyone whom they have reasonable grounds to suspect of being guilty of it. However, the constable must also have reasonable grounds to believe due to one of the below reasons it is necessary to arrest:

- Allow a prompt and effective investigation
- Prevent causing physical injury to themself or another
- Prevent suffering physical injury
- Prevent loss of or damage to property
- Protect a child or vulnerable person
- Ascertain a person's name
- Ascertain a person's address
- Prevent an offence against public decency
- Prevent causing an unlawful highway obstruction
- Prevent disappearance of suspect

This is commonly remembered by the pneumonic IDCOPPLAN

I - Investigation (to allow prompt and effective of the offence/conduct of the person)
D - Disappearance - prevent hindrance of prosecution by disappearance of suspect
C - Child (Protect child/vunerable other)
O - Obstruction of highway (prevent)
P - Physical injury (prevent)
P - Public decency (prevent commission of offence against)
L - Loss/damage to property (prevent)
A - Address (of suspect not known/doubted)
N - Name (of suspect not known/doubted)