The forces said the decision would protect frontline services
Witness protection units of four police forces are to merge to reduce costs.
Combining the three units run by Surrey, Hampshire, Sussex and Thames Valley Police, is expected to save at least £277,000 a year.
The forces said the change - that also applies to covert operations - would enable staff to provide better service and share equipment.
The move could lead to a reduction in posts, Sussex Police Authority admitted.
But a spokeswoman for the authority said these would be back office staff and the changes would help protect frontline services.
Peter Williams, chairman of Surrey Police Authority, said the move has been prompted by an expected "tougher" economic climate.
"Collaborating with our neighbouring authorities is a big part of those preparations," he said.
"The significant cost savings we expect to make through collaboration will free up money to help us protect the front line services that the people of the South East tell us matter most to them."
Councillor Jacqui Rayment, chair of Hampshire Police Authority, accepted that because the work is "carried out behind the scenes" the public would not see anything different.
"[But] they should be reassured that the work is being carried out to a higher standard, and that it is being done in a way that saves taxpayers' money," she said.
"Criminals do not recognise county borders when they are trying to escape justice, and this legal agreement sends a strong message that we are working together... our efforts will not stop because criminals live in a different county."
The agreement was signed by the four police authorities and four Chief Constables at a meeting in Sussex.