A system and method is described for well intervention in subsea installed oil and gas wells, comprising a surface vessel (10), or rig, with equipment (22) for handling and controlling a connection string (20) for downhole tools, and also a system (26) for the supply and return of drilling fluid, from where the connection string (20) for the downhole tool runs down in an actual drilling hole (36) of a well on the subsea, where a X-mas tree (12) with associated blowout preventer (16) is arranged on the well, and where a return line (24) for drilling fluid runs up to said system (26) on the surface vessel or the rig. The connecting string (20) for downhole tools runs into the well through open sea without a riser or landing string being fitted, and a removable intervention valve (14) is arranged in the drilling hole (36), where the intervention valve is arranged to function as a testable, temporary barrier.