The UK’s Serious Fraud Office has opened a criminal investigation into Tata Steel's British operations.
Police officers are investigating allegations staff tampered with certificates detailing the composition of the steel products before they were sold. The company's Yorkshire site is the focus of the inquiry, a media house reported.
At least nine employees have been suspended and around 500 clients, including BAE and Rolls-Royce, are thought to be affected.
There is also a trading standards investigation, but it is unclear whether it is linked to the criminal investigation.
Tata Steel announced it had put its entire UK business up for sale following a board meeting in Mumbai, putting thousands of workers' jobs at risk.
The company said the move was to stem heavy losses from high manufacturing costs, competition with China and a global oversupply of steel.
Ratan Tata, the former chairman of Tata Group, defended the decision, calling the UK steel operation "underinvested and overmanned".
In January, Tata announced more than 1,000 job losses at UK plants, 750 of them in Port Talbot.
That came after 1,200 job losses at Tata plants in Scunthorpe and Lanarkshire last October, at the same time steel manufacturer SSI closed another plant at Redcar.
Liberty House owner Sanjeev Gupta has emerged as the most likely buyer of Tata Steel UK.
The tycoon, who has already saved several UK steelworks, said the deal would require a partnership with the Government.
Business secretary Sajid Javid has said Tata Steel intends to formally begin the sales process by Monday 11 April "at the latest".