A method is for establishing an annulus barrier in a subterranean well. The method comprises providing a plug in the well and along a longitudinal section thereof, wherein the plug, at least in a portion of the longitudinal section, covers substantially the entire cross-section of the well in such a manner that the plug covers both the inside and the outside of a casing; removing a central through portion of the plug internally in the casing in such a manner that a through central opening is formed in the plug, and in such a manner that at least a cross-sectional section of the plug remains on the outside of the casing; disposing and anchoring a connection pipe in the well, and internally in the casing, in such a manner that the connection pipe extends at least along a length of the remaining cross-sectional section; and sealing, in a fluid-tight manner, an annulus between the casing and the connection pipe.