The one-loop finite temperature effective potential of QED in an external
electromagnetic field is obtained using the worldline method. The general
structure of the temperature dependent part of the effective action in an
arbitrary external inhomogeneous magnetic field is established. The
two-derivative effective action of spinor and scalar QED in a static magnetic
background at T=0 is derived.Comment: REVTeX, 5 pages. Minor corrections. Final version published in Phys.
Lett.