The Financial Reporting Council (FRC) says it will now look into work on the collapsed contractor’s 2013 accounts but it did not say why it has widened its timeline or when it expects to finish the probe, opened just weeks after Carillion went into administration last January.

The regulator declined to comment on why the investigation had been extended for the second time this year, or when it expected to conclude the probe into one of the most high profile corporate collapses of the past decade.

It was already looking into KPMG’s sign-off on Carillion’s books for 2014-2016 and the first half of 2017, and probing the outsourcer’s former finance directors, Richard Adam and Zafar Khan.

KPMG audited Carillion since 1999 and signed off on its accounts in March 2017 — four months before the business issued a catastrophic profit warning triggered by an £845 million writedown, and nine months before the company collapsed.

KPMG has also come under fire for breaching ethical standards in its work with Ted Baker and misconduct related to insurance claims handler Quindell.

The FRC is to fine it £4m for supervising the Cooperative Bank’s accounts before a £1.5 billion capital shortfall was found.

KPMG said: “We will continue to co-operate fully with the FRC’s investigation.”