An underwater flow retaining device (100,200,300,400,410,420,500) for retaining and blocking the outflow of a pressurized fluid from an outlet mouth of a tubular extension of an underwater well, said device comprising: - a flow-retaining head that is adapted to engage with said outlet mouth preventing said pressurized fluid from flowing out of said tubular extension, said flow-retaining head having a lower portion that in use faces said outlet mouth and an upper portion (10") opposite to said lower portion; - a support of said flow-retaining head; - a fastening means for fastening said support to said tubular extension; - an actuation means for carrying out a sliding movement of said flow-retaining head with respect to said support along a slide axis of said support towards said fastening means when said support is fixed to said tubular extension by said fastening means, between: - a manoeuvre position, in which said flow-retaining head is at a predetermined distance from said outlet mouth and said fastening means is fixed to said tubular extension, and - a retaining position, in which said flow-retaining head engages said outlet mouth, said distance of said manoeuvre position selected in such a way that said support is fixed to said tubular extension by said fastening means while said flow-retaining head is still at a position where it is not affected by said outflow of said pressurized fluid, and in that it has a channel that extends within said flow-retaining head between a lower passage port that is made at said lower portion and an upper passage port that is made at said upper portion (10"); characterized in that it further comprises a feed pump of a plugging fluid that can be arranged on a surface structure of the well, wherein said feed pump is hydraulically connected via a feed means and via said upper port with said channel of said flow-retaining head, and is arranged to transfer such a plugging fluid into said tubular extension of the well through the lower passage port in order to plug said well. - a flow-retaining head that is adapted to engage with said outlet mouth preventing said pressurized fluid from flowing out of said tubular extension, said flow-retaining head having a lower portion that in use faces said outlet mouth and an upper portion (10") opposite to said lower portion; - a support of said flow-retaining head; - a fastening means for fastening said support to said tubular extension; - an actuation means for carrying out a sliding movement of said flow-retaining head with respect to said support along a slide axis of said support towards said fastening means when said support is fixed to said tubular extension by said fastening means, between: - a manoeuvre position, in which said flow-retaining head is at a predetermined distance from said outlet mouth and said fastening means is fixed to said tubular extension, and - a retaining position, in which said flow-retaining head engages said outlet mouth, said distance of said manoeuvre position selected in such a way that said support is fixed to said tubular extension by said fastening means while said flow-retaining head is still at a position where it is not affected by said outflow of said pressurized fluid, and in that it has a channel that extends within said flow-retaining head between a lower passage port that is made at said lower portion and an upper passage port that is made at said upper portion (10"); - a manoeuvre position, in which said flow-retaining head is at a predetermined distance from said outlet mouth and said fastening means is fixed to said tubular extension, and - a retaining position, in which said flow-retaining head engages said outlet mouth, said distance of said manoeuvre position selected in such a way that said support is fixed to said tubular extension by said fastening means while said flow-retaining head is still at a position where it is not affected by said outflow of said pressurized fluid, and in that it has a channel that extends within said flow-retaining head between a lower passage port that is made at said lower portion and an upper passage port that is made at said upper portion (10");