The Big Four accounting firms continue to maintain a stronghold over the FTSE 100 audit market, the financial regulator has revealed. In its fourth report on the audit market, the Financial Reporting Council figures show 99 of the FTSE 100 are still audited by either PricewaterhouseCoopers, KPMG, Deloitte or Ernst & Young, with one mid-tier firm - BDO - entering the playing field this year. Concerns have been raised by some in the sector that should one of the top players leave the arena, audit choice could be further compromised if more firms aren't in the market. Outgoing FRC chief executive Paul Boyle, said the regulator remains 'concerned about the significant uncertainty and cost' that could arise if this were the case. 'Regardless of the actions taken by market participants,' he said, 'this risk is likely to remain significant in the medium to long term. It remains to be seen whether market-led actions will prove to be sufficient to reduce this risk to an acceptable level.' The FRC established a market participants group three years ago to propose changes to the audit market surrounding ownership and liability, but progress has so far been slow.