Private security guards will take over from police officers to operate cordons in the Amesbury Novichok inquiry.
Detectives are investigating the murder of Dawn Sturgess, 44, and the poisoning of her partner, Charlie Rowley, 45.
Wiltshire Police has been relying on so-called Mutual Aid, where other forces supply staff to help another constabulary which is under-strain from a big inquiry.
This is the second major operation in recent months, with Wiltshire Police helping the Counter Terrorism Policing Network investigate the poisoning of former Russian agent Sergei Skripal and his daughter Yulia earlier this year.
Officers were called to Amesbury after Ms Sturgess and Mr Rowley fell ill earlier this month.
A number of locations across Amesbury and Salisbury remain closed to the public, and officers have been standing guard 24/7 ever since.
Wiltshire's Chief Constable Kier Pritchard said the operation "might start to look slightly different when private security guards will join my officers on some of the cordons."