1. A tachyarrhythmia detection and classification system, comprising: a cardiac sensing circuit configured to sense at least one cardiac signal; a rate detector configured to detect a heart rate from the at least one cardiac signal; a tachyarrhythmia detector configured to detect a tachyarrhythmia episode using the heart rate; a physiological sensor configured to sense a physiological signal; and a morphology-based tachyarrhythmia classification module including: a feature extractor configured to extract arrhythmic morphological features of an arrhythmic heart beat of the detected tachyarrhythmia episode from the at least one cardiac signal; a similarity analyzer configured to compute a measure of similarity between the arrhythmic morphological features and stored template morphological features of a template heart beat of a known type cardiac rhythm; a classifier configured to classify the arrhythmic heart beat using the measure of similarity and a dynamically adjustable beat classification threshold (T); and a threshold generator configured to adjust the beat classification threshold (T) using an arrhythmic heart rate and an estimated normal heart rate range, the arrhythmic heart rate being the heart rate detected from the at least one cardiac signal during the detected tachyarrhythmia episode, the estimated normal heart rate range being a normal heart rate range estimated using at least the physiological signal.