How to solve a fourth order PDE with ndsolve?

I tried

NDSolve[{Laplacian[Laplacian[w[x,y],{x,y}],{x,y}]==P[x,y],bc},w,{x,0,1},{y,0,1}], but it says NDSolve::femcmsd: The spatial derivative order of the PDE may not exceed two.

How to derive the solution?