Questioned by an uneasy Commons Home Affairs Select Committee as she published the results of a month-long survey, Mrs Mills laid heavy emphasis on police responsibility for providing evidence.
The analysis of the halting of more than 10,000 cases in November last year shows that 43 per cent were stopped on grounds of insufficient evidence to provide a realistic prospect of conviction.
Asked whether she was satisfied with the quality of evidence from the police, she said: 'We wouldn't be discontinuing for unrealistic prospects of conviction if we were satisfied; 43 out of 100 are turned away so we are not satisfied.'
Sir Ivan Lawrence, a barrister and the Tory chairman of the committee, said he was concerned that the numbers of halted cases had doubled in the last five years.
A defensive Mrs Mills commented that it was 'difficult to win'. It was possible that in its early years the CPS was not reviewing cases as thoroughly. Too lax case reviews would risk allegations of potential miscarriages of justice.