Special Branch units across the South West are to join forces to create a borderless police team which can operate from Bristol to the Isles of Scilly.
All five forces are now set to collaborate in a bid to cut costs in the face of large budget reductions after chief constables and police and crime commissioners (PCC) agreed the move.
Officers from Devon and Cornwall, Avon and Somerset, Dorset, Gloucestershire and Wiltshire police forces will unite under shared management for investigative and intelligence work, including counter terrorism and domestic extremism.
Devon and Cornwall PCC Tony Hogg said: "Although we live in one of the safest parts of the country, we must never become complacent.
"The attempted bombing in Exeter five years ago proved that we have to constantly review threats and our response to them.
"The creation of a regional Special Branch unit, combining the experience of officers from five forces, increases our resilience and ensures that we continue to feel safe in our local communities".
The official signing of the deal to create the Special Branch collaboration took place at the South West regional meeting of PCCs and chief constables yesterday.
It is hoped it will enhance the highly specialist protection provided by Special Branch to give a more effective and efficient service.
The aim is to bring costs down without any reduction in the level or quality of service, in terms of management of threat and risk.
Nigel Rabbitts, chairman of the Devon and Cornwall Police Federation, the officers' staff association, said it was simply the formalisation of already existing joint working practices.
"There has always been a degree of collaboration within the Special Branch world," he added. "With reduced resources it is probably the way forward – in an ideal world we wouldn't have it but staff don't really have a strong view on the matter.
"It is a result of the cuts but certainly the small forces in our region need the support of the bigger ones to maintain their capability in counter-terrorism."
Nick Gargan, chief constable of Avon and Somerset Constabulary, said: "Neither serious organised criminals nor major transport routes are contained within force boundaries," he added.
"In times of austerity our ability to flex highly specialist services such as Special Branch to destroy and disrupt serious criminality becomes even more pressing."
Sue Mountstevens, PCC for Avon and Somerset, said it would enable resources to be focused in other areas such as neighbourhood policing