The technical field is that of pre-filled disposable needleless syringes, operating with a gas generator (100) and used for intradermal, subcutaneous and intramuscular injections of liquid active principle for therapeutic purposes in human and veterinary medicine. The inventive needleless syringe (1) is characterised in that it comprises a compact architecture body (2) and the gas generator (100) is released by a cap (17) covering said body (2), said cap (17) being adapted to slide along said body (2). Said releasing mode provides the syringe with great stability (1) at the time of injection.