Britain's Serious Fraud Office on Friday launched a probe into steelmaker GFG Alliance, focusing partly on links with its collapsed financier Greensill.

GFG Alliance, owned by Indian-British billionaire Sanjeev Gupta, had been Greensill's biggest customer at the time of the finance giant's notorious collapse.

GFG said it would cooperate fully with the investigation.

Greensill's spectacular implosion threatens 50,000 jobs at companies around the world that relied on its financing for their supply chains, including GFG.

It also has rekindled debate on close ties between the upper echelons of British politics and finance, with former prime minister David Cameron quizzed by lawmakers on his Greensill lobbying role this week.

"The SFO is investigating suspected fraud, fraudulent trading and money laundering in relation to the financing and conduct... of companies within the Gupta Family Group Alliance, including its financing arrangements with Greensill Capital UK Ltd," the SFO said in a brief statement on Friday.

It added that no further comment would be made on the live investigation.

In a separate statement, GFG Alliance said it "is making progress in the refinancing of its operations which are benefitting from... the very strong steel, aluminium and iron ore markets".

'Potentially criminal' allegations

Britain's City watchdog the Financial Conduct Authority earlier this week launched its own Greensill probe, revealing that "potentially criminal" allegations had been made about the circumstances of its collapse.

The Greensill affair shone a light on Gupta's own criticised business practices, with the UK government describing the GFG structure as "very opaque" after declining to rescue it.

Greensill Capital, which bypassed strict regulations forced upon traditional banks, specialised in short-term corporate loans via a complex and opaque business model that ultimately sparked its declaration of insolvency in March.

GFG has operations in more than 35 countries across the world, and annual global revenues of about $20 billion according to its website.

The group has 35,000 staff worldwide, including 5,000 in Britain where its Liberty Steel division is based.

Friday's news came one day after Cameron insisted he acted appropriately in controversial lobbying for Greensill.

British lawmakers questioned him following months of scandal and revelation about his lobbying ahead of the company's collapse.

The former Conservative leader, who was an adviser to Greensill and reportedly held lucrative stock options that are now worthless, maintains he was acting in the public good.

"I really believed in the solution that we had and we were putting to government that I thought would make a difference," Cameron told the virtual Treasury Committee hearing, in his first public appearance addressing the scandal.

"I would never put forward something that I didn't think was absolutely in the interests of the public good," he said, adding he was motivated to provide a solution for small business during "exceptional times".

'Damaging' lobbying claims

Cameron, in power from 2010 to 2016, faced a series of damaging claims he improperly and excessively lobbied former government colleagues seeking support for the stricken London-headquartered company early in the pandemic.

Documents showed this week that Cameron and his office staff last year sent ministers and officials 45 emails, texts and WhatsApp messages relating to Greensill, bypassing official channels.

Finance minister Rishi Sunak has previously said he "pressured" his staff to look into Cameron's requests, but insisted they independently assessed the proposals and ultimately rejected them.

Prime Minister Boris Johnson last month ordered a senior lawyer to investigate the entire episode.

Australian banker Lex Greensill, the founder of the bankrupt financial company, appeared Tuesday before the same committee and insisted he took "full responsibility" for its collapse.

"To all of those affected by this, I am truly sorry," Greensill said.

He also laid blame at insurer Tokio Marine, which withdrew cover to loans issued to Greensill clients amid the coronavirus pandemic.