Previous studies have reported the relationship between high maths anxiety and poor maths achievement but the precise aspects of maths and the circumstances under which maths anxiety would have a negative effect on performance remained debatable. The present study set out to investigate the effects of maths anxiety on maths performance under both timed and untimed conditions using complex and simple arithmetical problems (with and without carry or borrow operation respectively). University students with high levels of maths anxiety performed significantly poorer than those with medium or low maths anxiety in complex but not simple addition and subtraction problems. This interaction between maths anxiety and problem complexity remained significant even when test anxiety levels were partialled out. There was no evidence to support time pressure as a source of anxiety. Our findings provide the first report of a detrimental effect of maths anxiety on arithmetic performance in adult participants, having controlled for test anxiety.