The pilot completed his pre-takeoff checks before launching for a local flight. He subsequently advised that while he was manoeuvring behind a glider ahead of him in the circuit the canopy rattled and flew open. He was unable to close the canopy and shortly afterwards it tore free from the glider structure. A successful and uneventful landing was then carried out. Investigation revealed that it was possible to close the canopy locking handle fully without the locking pins engaging in the lock studs. In this type of glider the unsecured canopy was then not easy to detect from inside the cockpit. The pilot had limited experience on type and was not aware of the difficulty in detecting an unsecured canopy. Physical checks for canopy security by pushing on the canopy were discouraged by the gliding club for fear of damage. The launching crew had not noticed that the canopy was unlocked during the hookup procedures.