A barrier suitable for installation at least partly in a sewage pipe and for allowing a sewage flow within the sewage pipe to pass the barrier in a first direction of the sewage pipe, the first direction being a downstream direction, and for preventing rats or other vermin from passing the barrier in a second direction being opposite to the first direction upon installation of the barrier in the sewage pipe, the second direction being an upstream direction, said barrier comprising; - a support structure with a first hinge and a second hinge, - a first shutter pivotably mounted on the first hinge, - a second shutter pivotably mounted on the second hinge, and - a locking mechanism comprising - a first part of the locking mechanism formed as a separate part, the first part being a blocking mechanism pivotably mounted on, or near, the first hinge of the first shutter, the blocking mechanism being positioned between, or partly between, the first and the second shutter, and - a second part of the locking mechanism, the second part being formed as a protrusion from, or near, the top of the second shutter, wherein the barrier is arranged for being installed in the sewage pipe so that the first shutter and the second shutter are spaced apart along the first, downstream direction so that the first shutter is placed before the second shutter on an axis parallel with the first, downstream direction, and wherein the locking mechanism is provided for keeping the second shutter between a first, closed position (v) and a second, closed position (x) when the second shutter is tampered with by a vermin trying to pass the barrier from the second, upstream direction, the second shutter being allowed to have a limited rotation defined by engagement of said first part and said second part of the locking mechanism, said first part and said second part being arranged for engagement between, or above, the first and the second shutter, said rotation thereby being limited by the locking mechanism so that the second shutter cannot rotate to a third, open position (w), wherein, when the first shutter is in a closed position - the second part of the locking mechanism is capable of passing the first part of the locking mechanism so that the second shutter is rotatable from the third, open position (w) to the first, closed position (v). - a support structure with a first hinge and a second hinge, - a first shutter pivotably mounted on the first hinge, - a second shutter pivotably mounted on the second hinge, and - a locking mechanism comprising - a first part of the locking mechanism formed as a separate part, the first part being a blocking mechanism pivotably mounted on, or near, the first hinge of the first shutter, the blocking mechanism being positioned between, or partly between, the first and the second shutter, and - a second part of the locking mechanism, the second part being formed as a protrusion from, or near, the top of the second shutter, - a first part of the locking mechanism formed as a separate part, the first part being a blocking mechanism pivotably mounted on, or near, the first hinge of the first shutter, the blocking mechanism being positioned between, or partly between, the first and the second shutter, and - a second part of the locking mechanism, the second part being formed as a protrusion from, or near, the top of the second shutter, wherein the barrier is arranged for being installed in the sewage pipe so that the first shutter and the second shutter are spaced apart along the first, downstream direction so that the first shutter is placed before the second shutter on an axis parallel with the first, downstream direction, and wherein the locking mechanism is provided for keeping the second shutter between a first, closed position (v) and a second, closed position (x) when the second shutter is tampered with by a vermin trying to pass the barrier from the second, upstream direction, the second shutter being allowed to have a limited rotation defined by engagement of said first part and said second part of the locking mechanism, said first part and said second part being arranged for engagement between, or above, the first and the second shutter, said rotation thereby being limited by the locking mechanism so that the second shutter cannot rotate to a third, open position (w), wherein, when the first shutter is in a closed position - the second part of the locking mechanism is capable of passing the first part of the locking mechanism so that the second shutter is rotatable from the third, open position (w) to the first, closed position (v).