A downhole packer for providing a seal in a well bore to allow integrity testing of well bore with drill ahead capability immediately thereafter has a disengageable packer assembly wherein the packer element may be rendered disengageable by mounting the packer to the string using a tool body (61) provided with a sleeve (62) bearing a packer element (55), wherein the body is initially restrained from movement within the sleeve by engagement of an internal selectively movable retaining element (64). A method of testing a well bore with follow on drilling after disengaging the packer element is described.