Councils must give a year’s notice of leaving, so their move will have little practical effect.
Worthing leader Keith Mercer (Con) said: “SEERA has failed to deliver a change of heart on the part of the present government to plan for and provide adequate infrastructure for the south east over the next 10 years.
“By indicating our determination to leave SEERA now, we hope to send a clear signal to government that locally elected councillors are deeply unhappy with its inaction.”
Adur leader Neil Parkin (Con) said: “We felt it was no longer worth being in a government organisation we never wanted set up in the first place.
“It has cost taxpayers money to be in SEERA, whose recommendations and findings have been ignored by the government.”
The two councils work together closely and share a chief executive and senior management team.
Assembly chairman Keith Mitchell (Con) said he regretted the decision.
"I do not understand why elected councillors are willingly giving up their voice in the future of the region. Surrendering their democratic seat at the table is tantamount to backing government proposals that would hand key planning decisions to a body of unelected, government-appointed placemen.
“The assembly’s work on regional planning will continue until at least 2010 and we need to keep up the pressure on key issues such as house-building, protecting the green belt and investing in our pressured infrastructure. These problems will not go away after 2009 and Adur and Worthing’s residents will be without a powerful voice if their councils walk away from the table.”