Deloitte has been fined more than ยฃ6 million by the accounting watchdog over its audits of transport firm Go-Ahead.

The Financial Reporting Council (FRC) said it found a โ€œhighly concerning pattern of failureโ€ in the audits over a five-year period.

The regulator launched an investigation four years ago, a year after Go-Aheadโ€™s Govia joint venture with French firm Keolis was stripped of the Southeastern rail franchise by the Government for a โ€œserious breach of trustโ€.

On Thursday, the FRC said that there were โ€œnumerous breachesโ€ of audit requirements between 2016 and 2020.

It found that Deloitte failed to challenge two of Go-Ahead subsidiaries wrongfully retaining more than ยฃ30 million of public money for an extended period.

The FRC found that Go-Ahead had kept the money and released it as profit rather than returning it to the Government.

Go-Ahead stopped the practice after this was discovered by the Government, leading to a ยฃ23.5 million fine at the time.

The regulator would have fined Deloitte ยฃ11 million but reduced this to ยฃ6.05 million because of co-operation and admissions by the firm.

Penrose Foss, executive counsel and executive director of investigations and enforcement at the FRC, said: โ€œThese breaches show a highly concerning pattern of failure by Deloitte to apply sufficient scrutiny to decisions and actions by GAG (Go-Ahead Group) which were clearly questionable.

โ€œThe fact that some of those decisions and actions put very large amounts of UK taxpayersโ€™ money at risk is particularly troubling, and this is reflected in the high level of financial sanction imposed.โ€

A Deloitte UK spokeswoman said: โ€œWe regret that aspects of our audit work did not meet the standards expected and have learned from this matter.

โ€œWe are committed to continuous improvement and the delivery of high-quality audits.โ€

On Thursday, the FRC also closed a separate investigation into Deloitte related to financial statements for retailer Joules.

The watchdog confirmed that it had decided not to bring any enforcement action after reviewing evidence.