Deloitte is facing questions over its failure to spot accounting irregularities leading to a 'black hole' in the accounts at the RSA Insurance Group which saw chief executive Simon Lee resign earlier this month.

PwC is currently undertaking a comprehensive review of problems in the claims and finance functions at RSA's Irish business, amid concerns there could be a £70m write-down as a result of the irregularities which have not been specified.

Four of RSA's largest 15 institutional investors now want explanations from Deloitte and one has called for an investigation by the Financial Reporting Council (FRC).

According to the FT a top 10 investor in the company said: 'Deloitte need to explain why they missed this. This is not a matter for a formal investigation by the FRC at this stage. But it could come to that, if there are not answers.'

In a statement Deloitte said: 'We are no longer auditor to RSA and it would not be appropriate to speculate on the detail behind recent statements. Deloitte is committed to delivering the highest quality audit services.'

The firm, which has audited RSA since 2007, is already facing questions over the level of fees it has received for non-audit work for the insurance company. RSA's latest annual report shows the company paid Deloitte £10m in 2012 for management consulting as well as tax and other advice, compared with less than £6m in auditing fees.

When commenting on the proposed change in auditors in its results statement in February, RSA said 'perception' of the independence of Deloitte as auditor could be impaired after the firm was appointed to conduct further consultancy work on RSA's technology restructuring at its Scandinavian business, following an initial engagement last year.

The FRC said no formal investigation was under way. Deloitte is to be replaced by KPMG for RSA's audit, after the insurer put its external audit contract - worth £15.7m in 2012 - out to tender in 2013.