The invention concerns a method for synchronising clocks of base transceiver stations in a telecommunications system and a mobile communications system. According to the invention, in either some or in all base station sites of the telecommunications network a location measurement unit is installed, which receives an accurate clock including an extra-system signal. The clock information is transmitted further to base transceiver stations, whose frame transmissions are synchronised in accordance with the clock information. By using the implementation in accordance with the invention both economic and functional advantages are achieved, e.g. the capacity can be increased by up to 50-300%, depending on the manner of implementation.