Accountancy firm Grant Thornton is being investigated over its auditing of Patisserie Holdings.
The Financial Reporting Council (FRC) said it is investigating audits by Grant Thornton of three years – 2015, 2016 and 2017.
Last month Chris March, former finance director at the cafe chain Patisserie Valerie, was arrested after accounting irregularities were uncovered.
Mr Marsh is also being investigated by the FRC.
Mr Marsh is an accountant and member of the accountancy body the ICAEW.
The FRC said that under its rules Mr Marsh would be investigated for misconduct while the investigation into Grant Thornton would cover breaches of its rules.
A spokesman for Grant Thornton confirmed it had received a letter from the FRC about the decision to start an investigation. “We will, of course, fully cooperate in this matter,” the spokesman said.
It is the first time that the focus had been put on three years of the company’s accounts.
In October the Serious Fraud Office said it had opened an investigation into an individual, assumed to be Mr Marsh.
Mr Marsh, who has not been charged, was initially suspended and resigned shortly afterwards.
- Patisserie Valerie chairman Luke Johnson faces investor fury
Patisserie Holdings was saved from collapse when Luke Johnson, who owns 37% of the company, offered up to £20m in loans to the struggling chain.
Shareholders in the 206-store chain approved the rescue plan earlier this month despite complaining about the terms being offered to them.
Last week, Paul May, the chief executive of the company, which also owns Druckers, Philpotts, Baker & Spice and Flour Power City, was replaced by Stephen Francis.