The NHS cancer screening programme picked up around 6,000 invasive breast cancers that were missed by a manual check.
Tumours too small to be detected by hand were found when the patients were screened, official figures show.
The NHS Information Centre said 5,913 cancers smaller than 1.5 cm were found during England's breast screening programme in 2009-10.
The total comprises two-fifths (41.6%) of the 14,229 breast cancers found in over-45s during the period.
A study earlier this month said nearly a third of all breast cancers are found between regular screening sessions.
Usually they are detected because the woman notices symptoms such as a lump in the breast.
The so-called interval cancers either grow between the patient's appointments or are simply undetectable by a mammogram.
Experts behind that UK-wide study, published in the British Journal of Cancer, said reducing the length of time between mammograms will likely mean more cancers are detected.
Women in the UK aged 50 and over are invited for NHS breast screening only every three years. Around 1.5 million women are screened annually, although the programme is being extended to women aged over 47.
The NHS Information Centre's Tim Straughan said: "This highlights the vital impact of the national screening programme in detecting breast cancers that are so small they measure less than 15 mm and could have otherwise gone unnoticed."