The alliance was due to end this week, but the Warwickshire force asked the Government for more time to prepare.
Miss Patel has now written to West Mercia and Warwickshire Police forces and their respective Police and Crime Commissioners outlining her reasons.
The letter stated: "It is my understanding that you have not reached an agreement to smoothly transition services past October 8. If the collaboration agreement is allowed to terminate without agreed transition terms, I consider that there would be a severe operational impact on Warwickshire Police which would create a risk to the public that I am not prepared to tolerate.
"I therefore with immediate effect make the enclosed direction, which provides for the collaboration agreement to continue for a further six months. I have carefully considered the period of this extension and feel that a six-month extension provides sufficient time for both forces to reach agreement. It is clear to me that this is the best way to ensure the efficiency and effectiveness of policing in this case."
Home Office officials will work with to help identify independent expertise to assist the forces.
She added that she hoped a swift agreement will be made.
West Mercia Police Chief Constable Anthony Bangham said: “The extent to which Warwickshire relies on West Mercia to provide critical policing functions is clearly acknowledged by the Home Secretary. Despite the 12-month notice period, Warwickshire has not prepared sufficiently to operate as a standalone force and clearly that would place their communities at risk.
"While a short continuation of the alliance was anticipated, to continue the current arrangement for a further six months impinges on my operational independence as chief constable and significantly limits my ability to provide effective, efficient policing to the public of West Mercia.”
West Mercia Police and Crime Commissioner John Campion said: “A six-month extension to the alliance on its existing terms simply means our communities have to continue subsidising Warwickshire. This arrangement was terminated in the best interests of West Mercia’s communities, yet 12 months on we find ourselves still tied to an alliance that actively prohibits our police delivering the best possible services in West Mercia.
"Warwickshire have clearly and consistently stated their preference to transition to a standalone force, yet the public in West Mercia continue having to pay the bill for their failure to achieve that. However, we will continue working with Warwickshire, the Home Office, and other partners towards a smooth transition of services.”
The extension was made under section 23G of the Police Act 1996.