The issue was caused by the way the agency brought together logs produced by commercial firms paid to analyse swab tests of the public, to discover who has the virus.

They filed their results in the form of text-based lists - known as CSV files - without issue.
...
The problem is that PHE's own developers picked an old file format to do this - known as XLS.

As a consequence, each template could handle only about 65,000 rows of data rather than the one million-plus rows that Excel is actually capable of.

https://www.bbc.co.uk/news/technology-54423988