ACPO spokesperson said:
"As any police force may do, the PSNI are requesting mutual aid assistance because of operational challenges. We anticipate that an additional 30 public order units will travel to Northern Ireland, equating to about 600 police officers. These represent an additional resource for PSNI without in any way compromising capacity of police forces on the mainland to respond to events.
"These officers are well equipped to support colleagues in NI having trained with the PSNI in preparation for the G8. The PSNI may use them as a strategic reserve to deploy, if necessary.
"As with every operation, we will review afterwards to see what lessons may be learned."