The UK government’s top law officer should have a better idea by the end of next month of the extent of the problems at the Serious Fraud Office that led to the quashing of two convictions in one of the agency’s biggest corruption cases.

Attorney-general Suella Braverman is due to receive the results of a probe she ordered at the end of last year into the derailing of a bribery case involving energy consultancy Unaoil. Three judges had castigated the SFO in handing down their judgment in the first of two successful appeals.

Braverman gave barrister Sir David Calvert-Smith a sweeping remit to investigate those failings but also report back by the end of May with any changes he thinks are needed to fix the agency’s culture, policies and procedures.

The fiasco has put the spotlight on the agency’s director Lisa Osofsky and raised questions about her future. But critics of the SFO have warned that replacing its boss would not solve the deep-rooted issues at the agency.

Resource-constrained and heavily reliant on whistleblowers and corporate admissions of guilt, the SFO remains dogged by many of the same problems that first saw it dubbed the “Serious Farce Office” by satirical magazine Private Eye back in the 1990s.

“The SFO is looking like a sorry beast at the moment,” said Susan Hawley, executive director at anti-corruption group Spotlight on Corruption. “Some of the issues concern [Osofsky’s] leadership while others are chickens coming home to roost . . . The SFO desperately needs to regain its mojo.”

After her appointment in 2018, Osofsky, a dual US-UK national who cut her teeth as an assistant US attorney in Chicago, pledged to take a “drains up” look at the SFO’s caseload. She spoke of making more use of American tactics such as “flipping” defendants to become co-operating witnesses — a promise that was met with a mixture of hope and scepticism in the UK legal profession.

Hope turned to dismay after she was heavily criticised by a judge in the Unaoil trial in 2020 for allowing David Tinsley, an investigator working for the consultancy’s founding family, to become embroiled in the case. That criticism was reiterated by Court of Appeal judges late last year. The appeal found the agency had also withheld evidence, including sometimes toe-curling text messages between Osofsky and Tinsley.

Shadow attorney-general Emily Thornberry, who worked as a barrister before entering politics, told the Financial Times she was “surprised” that Braverman had let Osofsky remain in the post following the Court of Appeal ruling. But she said changing the SFO’s leadership or reviving plans to fold it into the National Crime Agency, which also fights economic crime, would “not by itself address the fundamental problem”.

She added in an email: “The UK needs an entirely new, modernised approach to tackling serious fraud . . . and simply shifting around a few personnel is not going to cut it.”

Thornberry echoed Osofsky in calling for more effective legislation to allow the SFO to prosecute corporate fraud.

But the SFO faces a range of other issues, including a limited budget, staff shortages, an unsustainable disclosure regime and an apparent risk aversion in bringing prosecutions.

One issue is the stark disparity between its own resources — its core annual budget is £53mn — and the almost unlimited means at the disposal of its targets, including FTSE 100 companies and oligarchs. According to estimates by the NCA, £100bn of “dirty money” flows through the UK each year.

On top of that, the agency struggles to retain the army of lawyers, digital experts and technologists needed to sift through the mountains of data involved in complex financial crime because it cannot compete with private sector pay rates.

That leaves it stretched thin when it comes to investigating and prosecuting complex cases, and heavily reliant on companies and whistleblowers to supply it with evidence.

“The number of cases being investigated and tried by the SFO has undoubtedly declined in the past few years,” said Stephen Parkinson, a former prosecutor who is now the senior partner at law firm Kingsley Napley.

“While it is true that there appear to be no major corporate scandals that are not being investigated . . . it is difficult to avoid the conclusion that it has suffered a loss of confidence, zeal and drive,” he added.

The SFO opened eight cases but closed 12 investigations, including several major probes, according to the agency’s latest annual report covering 2020-21.

The SFO said in response that “taxpayers wouldn’t thank us for keeping investigations open that have no realistic prospect of leading to a conviction”.

The agency is also having to deal with the more widespread problem of disclosure. Last year the SFO bungled disclosure in its case against two former Serco executives to such an extent that the judge directed the jury to acquit both men.

Disclosure is a growing problem for all law enforcement agencies. But the document-intensive nature of white-collar crime cases creates an acute challenge for the SFO, which employs junior barristers and small teams of investigators to handle the most crucial elements of a case.

The agency in response said it needed “criminal courts to recognise that technology can expedite disclosure” and called for new legislation that was “fit for today’s digital environment”.

Lawyers have warned there was also a structural issue with the use of a key tool designed to tackle corporate wrongdoing — a version of US-style plea bargains known as a deferred prosecution agreement introduced in 2014. Although these deals have generated £1.3bn in fines, they have not been accompanied by any individual prosecutions.

“I believe the SFO is gripped by a sense of defeated acceptance when it comes to corporate fraud — the idea that because it’s become increasingly difficult in the modern age to convict fraudulent companies or the people who run them, it’s a waste of effort to try,” Thornberry said.

Braverman will be hoping Calvert-Smith has the answers.