The Financial Reporting Council has extended its investigation into the Cup Trust tax-avoidance vehicle to include its auditor, Hillier Hopkins.
In December 2013, the FRC said it was investigating whether any chartered accountant linked with the Cup Trust could be guilty of misconduct.
The FRC said this week in a statement that it would extend the investigation "to include the preparation and audit of the financial statements for the years ending 31 March 2010 and 31 March 2011". It said that the audit for both of these years was carried out by Hillier Hopkins.
It said that it was already investigating whether "any member associated with the Cup Trust may have committed misconduct with respect to the establishment and operation of the charity and any related tax planning issues".
The Cup Trust was a charity established to avoid tax. In two years the scheme raised £176m in donations, claimed £46m in tax relief but spent only £55,000 on charitable activities.
The FRC sets accountancy standards in the UK, and anyone wishing to practise as a chartered accountant must be a member of a professional body signed up to the FRC accountancy scheme, such as the Institute of Chartered Accountants in England and Wales.
The scheme provides a framework for ensuring that if members do not conduct themselves properly in the course of their job, they may be subject to disciplinary action.
The FRC investigates individuals referred to it by others, and will open a formal inquiry if there is sufficient evidence. If that inquiry finds a case to answer, the matter will be taken to a formal tribunal made up of accountants and other professionals with relevant experience.
If that tribunal finds against the individual they can be reprimanded, fined, or have their membership of the accountancy profession suspended.
A statement from Hillier Hopkins said the company had been cooperating with the FRC. "We have not been informed of the nature of the extended investigation that the FRC is now conducting, beyond the information in its press notice," it said. "Consequently we have no further information on which to make comment at this time. We will continue to fully cooperate with the FRC."