A fastening system for fastening sandwich-structured composite panels to a support structure, the fastening system comprising - a sandwich-structured composite panel comprising a first face skin of sheet metal, a second face skin of sheet metal, and a core made of thermally insulating material arranged between the first face skin and the second face skin, the first and second face skins being attached to the core, and - a fastener for fastening the sandwich-structured composite panel to the support structure, the fastening system comprising an installation hole formed through the first face skin and the core, and an elongated sleeve insertable into the installation hole, the sleeve being made of non-metallic, galvanically isolating and thermally insulating material, the sleeve comprising a tubular portion having a first end, a second end, a hollow interior defined by a side wall of the tubular portion, a bottom wall at a second end and a collar flange encircling an open mouth at the first end, the sleeve having a length (L1, L2) extending in the installation hole from the first face skin towards the second face skin, whereby the sleeve forms an installation guide for the fastener which is insertable into the interior and fastenable through the bottom wall whereby, when the sleeve is in the installation hole, the collar flange abuts against an outer surface of the first face skin and the fastener being fastened to the support structure pressing the second face skin against the support structure, characterized in that the sleeve has a length corresponding to a combined thickness of the core and the first face skin, whereby when the sleeve is in the installation hole, the bottom wall abuts against an inner surface of the second face skin. - a sandwich-structured composite panel comprising a first face skin of sheet metal, a second face skin of sheet metal, and a core made of thermally insulating material arranged between the first face skin and the second face skin, the first and second face skins being attached to the core, and - a fastener for fastening the sandwich-structured composite panel to the support structure, the fastening system comprising an installation hole formed through the first face skin and the core, and an elongated sleeve insertable into the installation hole, the sleeve being made of non-metallic, galvanically isolating and thermally insulating material, the sleeve comprising a tubular portion having a first end, a second end, a hollow interior defined by a side wall of the tubular portion, a bottom wall at a second end and a collar flange encircling an open mouth at the first end, the sleeve having a length (L1, L2) extending in the installation hole from the first face skin towards the second face skin, whereby the sleeve forms an installation guide for the fastener which is insertable into the interior and fastenable through the bottom wall whereby, when the sleeve is in the installation hole, the collar flange abuts against an outer surface of the first face skin and the fastener being fastened to the support structure pressing the second face skin against the support structure, characterized in that the sleeve has a length corresponding to a combined thickness of the core and the first face skin, whereby when the sleeve is in the installation hole, the bottom wall abuts against an inner surface of the second face skin.