The agreement should see more money filter down to the second tier of the game in Scotland, the SPL said in a statement on Tuesday.
Many clubs are facing a struggle to survive financially in Scotland where the collapse of former champions Rangers has added to the sense of crisis.
The plan will retain the current structure of a 12-team top flight and three lower divisions each of 10 teams.
Scottish soccer has long been dominated by the Glasgow clubs Celtic and Rangers. The demise of Rangers, who had to relaunch from the fourth tier this season after their financial collapse, has further dented its commercial prospects.
To stimulate interest around promotion and relegation, the SPL clubs have proposed playoffs between the team finishing second from bottom in the top flight and those placed second to fourth in the tier below it.
SPL Chairman Ralph Topping believed the merger could be completed by August.
"Much work needs to be done in a short space of time to achieve our objective of a single merged league this summer," he said.
"But, where there is a will, there is a way. The time for action is now."
- Sports & Recreation