The Big Eight gradually reduced due to mergers between these firms, as well as the 2002 collapse of Arthur Andersen, leaving four networks dominating the market at the turn of the 21st century. In the United Kingdom in 2011, it was reported that the Big Four account for the audits of 99% of the companies in the FTSE 100 Index, and 96% of the companies in the FTSE 250 Index, an index of the leading mid-cap listing companies. Such a high level of industry concentration has caused concern, and a desire among some in the investment community for the Competition & Markets Authority to consider breaking up the Big Four. In October 2018, the CMA announced it would launch a detailed study of the Big Four's dominance of the audit sector. In July 2020, the UK Financial Reporting Council told the Big Four that they must submit plans by October 2020 to separate their audit and consultancy operations by 2024.</onlyinclude>