Port Talbot
Tata Steel's plant in the town of Port Talbot, Wales Rebecca Naden/Reuters

The Serious Fraud Office (SFO) has launched a criminal probe into allegations that staff working for Tata Steel's UK office may have tampered with certificates outlining the composition of the steel before they were sold.

The company's Yorkshire site is thought to be the focus of the inquiry, according to the Telegraph, who disclosed the revelations. Tata Steel reportedly referred itself following an internal audit where it was discovered that improper testing and certification may have taken place.

At least nine employees have been suspended. About 500 clients are thought to be affected, including Rolls-Royce. A spokesperson for the company said: "We were made aware of an issue by Tata last year. We have not been contacted by the SFO and cannot comment on any investigation."

Tata has put its UK steel operations up for sale, with thousands of workers facing the prospect of losing their jobs. Speaking in Washington on 7 April, former group chairman and Indian industrialist Ratan Tata said the "English facilities are underinvested [and] overmanned".

Tata's decision to sell up had caught the government off guard. Business Secretary Sajid Javid – who was in Australia when the announcement was made – said that while the government was aware that the company was reviewing its UK operations, it had gone "much further than we expected".

With its UK plants set to go up for sale on 11 April, Javid has assured union members that Tata Steel will be a "responsible seller". Commodities trading firm Liberty House is the only potential investor to have publicly declared an interest to buy parts of Tata's operations.

But concerns are growing over whether Sanjeev Gupta's Liberty House has the resources to save Port Talbot and other Tata Steel sites in the UK without significant financial support from the government. Gupta has insisted that nothing is for certain, adding "it is very early days."

Despite desperate efforts to save Britain's steelmaking industry and thousands of livelihoods, there is a gloomy outlook on Tata Steel's plants in the UK.

"Port Talbot has little future for its blast furnaces and will have to be ultimately shut down. Steelmaking in the UK under the current spread of steel and raw material makes no sense to us at all," said Macquarie analyst Rakesh Arora.

"The rolling business could have some takers but will require serious concessions from the government, mothballing of lots of facilities and large workforce layoffs. Who is going to assume the restructuring costs? We expect no new buyer will agree to it and the onus will fall either on Tata Steel or the UK government. We believe zero cost is the only best case outcome here for Tata Steel," he added.

Cardiff Business School professor Calvin Jones had a similar forecast and warned that closing the Port Talbot site could be a less painful alternative to a "slow death". "It is with absolutely no pleasure that I suggest that if Port Talbot is to close – and it will – it may be better done now when we can squeeze every remedial rupee possible out of a rich corporate owner," Jones wrote for the BBC.

He added: "The alternative will be the death of a thousand cuts for the next decade or more, with the public sector left to clean up the eventual mess, and in the meantime no one able to develop a positive, sustainable vision for Port Talbot."