The flight crew were conducting an ILS Category II approach and landing on runway 27 at Bristol Airport. On touchdown they noticed that the aircraft de-rotated sharply. The pilot flying (PF) was unable to maintain directional control during the landing roll and the aircraft ran off the left side of the runway onto the grass. At some point during the landing the throttles were moved forward, reducing the rate of deceleration. As the aircraft left the paved surface the crew realised that the landing had been carried out with the Emergency/Parking brake set. The aircraft may have remained on the runway surface but for the addition of forward thrust during the landing roll. Conclusion: The accident arose as a result of the inadvertent selection of the Emergency/Parking brake instead of the speed brake. The levers are of similar shape and sited close to each other but with a different appearance and mode of action. There is also a brake on indicator light. These risk controls proved ineffective in preventing the inadvertent selection of the Emergency/parking brake both on this occasion and on at least two previous occasions. Once the parking brake had been set there were opportunities to detect and correct the error, but a busy flight deck environment together with a high workload contributed to it going unnoticed. After touchdown, the aircraft may have remained on the runway surface but for the addition of forward thrust during the landing roll. The manufacturer stated that it did not intend to conduct a system review relating to the parking brake status.