LONDON (Reuters) - The UK’s Serious Fraud Office (SFO) has abandoned two of its largest and longest investigations, into alleged bribery at aero engine maker Rolls-Royce and drugs maker GlaxoSmithKline, in a move described by one lawyer as “extraordinary”.

A sign is displayed in an unmarked Serious Fraud Office vehicle parked outside a building, in Mayfair, central London March 9, 2011. Property tycoons Vincent and Robert Tchenguiz were among nine people arrested on Wednesday after police raided addresses in London and Reykjavik linked to an investigation into the collapse of Iceland's Kaupthing Bank. REUTERS/Andrew Winning (BRITAIN - Tags: CRIME LAW BUSINESS)

The decision, which comes under SFO Director Lisa Osofsky who took the helm last August pledging to review the agency’s caseload, weed out weaker cases and propel others forward, underlines the difficulties of prosecuting senior executives.

“After an extensive and careful examination, I have concluded that there is either insufficient evidence to provide a realistic prospect of conviction or it is not in the public interest to bring a prosecution in these cases,” Osofsky said.

Rolls-Royce paid almost 500 million pounds ($650 million) under a Deferred Prosecution Agreement (DPA) with the SFO in 2017, drawing a line under a four-year investigation into allegations of criminal conduct spanning three decades, at least seven jurisdictions and three of the company’s business sectors.

Rolls apologized unreservedly after it was found to have paid bribes including a luxury car and millions of pounds of cash to middlemen to secure orders in countries such as Indonesia, Russia, India, China and Nigeria.

But a parallel investigation into individuals associated with Rolls-Royce, and the inquiry into GlaxoSmithKline and its subsidiaries, which began in 2014 shortly after the drugs group was fined 3 billion yuan ($447 million) in China for paying bribes to doctors to use its drugs, had made little progress.

“It is extraordinary the SFO are unable to charge any individual suspects in relation to Rolls-Royce, given the scale of the allegations in the DPA,” said Sarah Wallace, a partner at law firm Irwin Mitchell.

HISTORICAL CASES

But Wallace added: “It looks like Osofsky is drawing a line under historical cases and wants to stamp her own mark on new cases going forward.”

Robert Barrington, a director at anti-corruption group Transparency International, said the decision to drop the Rolls-Royce case could send a message to companies that DPAs were a soft option that give impunity to those who break the law.

“It is absurd that yet again a company can admit to bribery and yet neither the bribe payers, nor the management team that allowed the crime to happen, are held responsible,” Barrington said.

Rolls-Royce said it had noted the decision and declined to comment further. GSK said it was pleased at the SFO decision and that no further action was required.

The SFO, which faced criticisms of serious failings last year after the collapse of a high-profile retrial of former executives from retailer Tesco, said Osofsky had quietly closed a number of investigations since taking up her post.

Jonathan Pickworth, a lawyer at White & Case, said she deserved credit for making overdue, tough decisions about longstanding cases.