New research has revealed that just under one in three breast cancer diagnoses are performed between regular screenings.
The development of symptoms – including a lump in the breast – is what prompts the cancer to be detected.
Experts at the Institute of Cancer Research (ICR) looked at the information on the number of breast cancers that are found between screenings, the first time that such data have been analysed.
The 'interval cancers' include tumours that were undetected or missed in mammograms, as well as those that developed in between appointments.
Researchers said that more cancers could be detected if the time between mammograms was shortened, but it may not have an effect on death rates.
Women in the UK aged 50 and over are invited for NHS breast screening every three years.
Around 1.5 million women are screened annually and the programme is in the process of being extended to all women aged 47 to 73.
Experts examined data from more than 7.3 million women aged 50 to 64 who had a routine screen between April 1997 and March 2003 at one of 92 screening centres in England, Wales and Northern Ireland.
The results showed that 2.91 per 1,000 women who were given the all-clear at their screening were diagnosed with breast cancer before their next mammogram was due.
Senior author Dr Sue Moss said: 'The rate at which women developed breast cancer between screenings was higher than expected, and is likely to reflect the fact that breast cancer rates are rising in general.
'Reducing the length of time between screenings would result in a lower proportion of interval cancers, but not necessarily a significant reduction in mortality.'
The research has been published in the British Journal of Cancer.