A method for establishment of an annulus barrier (51, 51') in a subterranean well, wherein the well, at least in a portion where the annulus barrier (51, 51') is to be established, is provided with a casing, and wherein the method comprises the following steps: providing a plug (25, 25') in the well and along a longitudinal section (L1, L1') thereof, wherein the plug (25, 25'), at least in a portion of the longitudinal section (L1, L1'), covers substantially the entire cross-section of the well in such a manner that the plug (25, 25') covers both the inside and the outside of the casing, step also comprising the following sub-steps: conducting a perforation tool down into the casing and to the longitudinal section (L1, L1') where the plug (25, 25') is to be set; forming holes in the casing along the longitudinal section (L1, L1') by means of the perforation tool; by means of a washing tool, which is attached to a flow-through string and is conducted down to the longitudinal section (L1, L1'), pumping a washing fluid down through the string and out into the casing via the washing tool; by means of a directional means connected to the washing tool, conducting the washing fluid out into an annulus between the outside of the casing and a surrounding formation; and pumping a fluidized plugging material down through the string and out into the casing, thus also into the annulus via the holes in the casing; removing a central through portion of the plug (25, 25') internally in the casing in such a manner that a through central opening is formed in the plug (25, 25'), and in such a manner that at least a cross-sectional section of the plug (25, 25') remains on the outside of the casing; disposing and anchoring a connection pipe (27, 27') in the well, and internally in the casing, in such a manner that the connection pipe (27, 27') extends at least along a length (L2, L2') of the remaining cross-sectional section; and sealing, in a fluid-tight manner, an annulus (26, 26') between the casing and the connection pipe (27, 27') along at least the holes formed in the casing, thereby ensuring the integrity of the casing. providing a plug (25, 25') in the well and along a longitudinal section (L1, L1') thereof, wherein the plug (25, 25'), at least in a portion of the longitudinal section (L1, L1'), covers substantially the entire cross-section of the well in such a manner that the plug (25, 25') covers both the inside and the outside of the casing, step also comprising the following sub-steps: conducting a perforation tool down into the casing and to the longitudinal section (L1, L1') where the plug (25, 25') is to be set; forming holes in the casing along the longitudinal section (L1, L1') by means of the perforation tool; by means of a washing tool, which is attached to a flow-through string and is conducted down to the longitudinal section (L1, L1'), pumping a washing fluid down through the string and out into the casing via the washing tool; by means of a directional means connected to the washing tool, conducting the washing fluid out into an annulus between the outside of the casing and a surrounding formation; and pumping a fluidized plugging material down through the string and out into the casing, thus also into the annulus via the holes in the casing; conducting a perforation tool down into the casing and to the longitudinal section (L1, L1') where the plug (25, 25') is to be set; forming holes in the casing along the longitudinal section (L1, L1') by means of the perforation tool; by means of a washing tool, which is attached to a flow-through string and is conducted down to the longitudinal section (L1, L1'), pumping a washing fluid down through the string and out into the casing via the washing tool; by means of a directional means connected to the washing tool, conducting the washing fluid out into an annulus between the outside of the casing and a surrounding formation; and pumping a fluidized plugging material down through the string and out into the casing, thus also into the annulus via the holes in the casing; removing a central through portion of the plug (25, 25') internally in the casing in such a manner that a through central opening is formed in the plug (25, 25'), and in such a manner that at least a cross-sectional section of the plug (25, 25') remains on the outside of the casing; disposing and anchoring a connection pipe (27, 27') in the well, and internally in the casing, in such a manner that the connection pipe (27, 27') extends at least along a length (L2, L2') of the remaining cross-sectional section; and sealing, in a fluid-tight manner, an annulus (26, 26') between the casing and the connection pipe (27, 27') along at least the holes formed in the casing, thereby ensuring the integrity of the casing.