British justice secretary Kenneth Clarke has sparked a furious argument over the definition of rape after saying some assaults are not as serious as others.

Clarke was discussing government proposals to halve the sentences of some rapists who plead guilty when he said he believed date rapes "vary enormously."

Opposition leader Ed Miliband called on Prime Minister David Cameron to fire Clarke for making a distinction among assaults.

Clarke told the BBC on Wednesday that judges already level different sentences based on the circumstances of the rape.

He says a teenage boy who had intercourse with a girl who is under the age of consent already receives a shorter sentence than a person who carried out "a serious rape, with violence and an unwilling woman."