Electrical connector system comprising a locking assurance device and a first connector housing configured for being engageable with a second mating connector housing that is capable of being locked therewith, wherein: the locking assurance device comprises at least one spring and is configured to be movably accommodated in the first housing such that a displacement of the locking assurance device in the first housing from a delivery position causes a loading of the said at least one spring; and the locking assurance device is further configured such that, during an action of mating the second housing into the first housing, the locking assurance device is displaced from the delivery position by the second housing such that the loading of the said at least one spring opposes the mating as long as the electrical connector system is not in a locking state in which the second housing is locked with the first housing, and the locking assurance device is not returned to the delivery position; wherein the locking assurance device comprises a body configured for being movably accommodated, in particular in a sliding manner, in the first housing in a mating direction of a second housing into the first housing; and wherein the first housing comprises a body comprising an accommodation portion which is a recess of geometry adapted to that of the locking assurance device; characterised in that the locking assurance device further comprises an unlocking thrust surface configured such that, in the locking state, a displacement of the locking assurance device from the delivery position by a thrust on the said unlocking thrust surface causes a loading of the said at least one spring authorizing the unlocking of the first and second housings (200, 400). the locking assurance device comprises at least one spring and is configured to be movably accommodated in the first housing such that a displacement of the locking assurance device in the first housing from a delivery position causes a loading of the said at least one spring; and the locking assurance device is further configured such that, during an action of mating the second housing into the first housing, the locking assurance device is displaced from the delivery position by the second housing such that the loading of the said at least one spring opposes the mating as long as the electrical connector system is not in a locking state in which the second housing is locked with the first housing, and the locking assurance device is not returned to the delivery position; wherein the locking assurance device comprises a body configured for being movably accommodated, in particular in a sliding manner, in the first housing in a mating direction of a second housing into the first housing; and wherein the first housing comprises a body comprising an accommodation portion which is a recess of geometry adapted to that of the locking assurance device; characterised in that the locking assurance device further comprises an unlocking thrust surface configured such that, in the locking state, a displacement of the locking assurance device from the delivery position by a thrust on the said unlocking thrust surface causes a loading of the said at least one spring authorizing the unlocking of the first and second housings (200, 400).