Britain's accountancy watchdog has slapped PwC with a £5 million fine for "misconduct" in relation to its 2009 audit of Connaught.
The Financial Reporting Council (FRC) said that PwC and Stephen Harrison, a retired audit partner at the firm, have been "severely reprimanded" following an investigation.
Mr Harrison has been fined £150,000.
The reprimands relate to PwC's audit of Connaught, a FTSE 250 company which went into administration in 2010.
The misconduct charge relates to three audit areas - mobilisation costs, long-term contracts and intangible assets.
PwC has also been ordered to pay the FRC's legal costs and to make an interim payment on account of £1.5 million.
PwC said in a statement: "We are sorry that our work fell short of professional standards.
"Since 2010 when the case began, we've worked hard to improve our procedures and processes. Audit quality is of paramount importance to PwC and the FRC's annual audit quality assessments have shown a trend of improvement in our work over several years."