A downhole disconnect device for disconnecting an intervention tool string from a wireline, includes: a tubular body having a first part directly or indirectly coupleable to the wireline and a second part connectable directly or indirectly to an intervention tool string; an actuator for disconnecting the second part of the tubular body from the first part of the tubular body; at least one sensor to detect when the tool string is stuck in use and to produce an output signal accordingly; a controller configured to receive the output signal and selectively operate the actuator accordingly; and a power source. The tubular body is adapted to house the actuator, sensor, controller and power source and the device further comprises a timer configured to delay operation of the actuator by a predetermined time on receipt of the output signal from the sensor. There is also a method of operating the downhole disconnect device.