A criminal probe was opened into activity at a business unit of Tata Steel, the Serious Fraud Office (SFO) has said.
The SFO did not give details about the nature of the investigation which has been confirmed at a difficult time for the steel industry.
In a statement, the SFO said: "The Serious Fraud Office confirms it opened a criminal investigation in December 2015 into activity at Speciality Steels, a business unit of Tata Steel (UK) Ltd. We can make no further comment at this time."
The news comes after Tata said they would sell all UK assets, sparking fears for jobs.
It comes on the same day as keys to steel mills at Dalzell and Clydebridge in Scotland were formally handed to new owners, heralding "a new era" for the factories which had been threatened with closure.
Scottish First Minister Nicola Sturgeon hailed international metal firm Liberty House's takeover from Tata Steel as a demonstration of the "art of the possible" at the handover ceremony in Motherwell, North Lanarkshire.
The plants were secured last month in a "back-to-back" agreement, involving the Scottish Government buying them from Tata Steel, and immediately selling them on to Liberty House.
Ms Sturgeon urged the UK and Welsh governments to step up efforts to save the giant Port Talbot steel works in South Wales.
Tata will begin the sale process for the site, which is the largest steel plant in the UK, on Monday.