A former MI6 boss and an ex-Liberal Democrat peer used retired intelligence officers to gather “sensitive” information from secret sources in the Romanian government to help a wealthy businessman facing corruption allegations, according to documents seen by the Observer.

Sir John Scarlett’s private intelligence firm, SC Strategy (SCS), which he co-owns with Lord Carlile, disclosed in a report produced for the businessman’s lawyers that it had obtained information from a range of well-placed “confidential sources” in Romania on sensitive issues including diplomatic negotiations and the activities of anti-corruption prosecutors.

The 2016 report suggests that SCS’s operatives enjoyed a remarkable degree of access in Bucharest and paid sources who had “direct and intricate knowledge” of Romania’s “political, security and intelligence apparatus”. One source is described as “a very high level” official in the president’s office. SCS undertook the intelligence work for Alexander Adamescu, a media and property tycoon who heads one of Romania’s wealthiest families.

Adamescu is awaiting extradition to Romania from the UK after Romanian prosecutors accused him of bribery and money laundering. The businessman has denied the allegations.

Lawyers for Adamescu filed the 36-page report as expert evidence in extradition proceedings held in London in 2017. Written by Carlile, who is a senior QC and former independent reviewer of terrorism legislation, the report bolstered the businessman’s claims that the Romanian prosecution is politically motivated.

The document provides a rare glimpse into the world of private intelligence and sheds light on the activities of two distinguished former public servants and their discreet, well-connected British company.

Responding to the Observer’s queries, Carlile insisted SCS used “entirely lawful means” to obtain legal information. “That we obtain useful information is a result of our knowing how to conduct effective investigations,” he said.

The peer categorically rejected any suggestion that either he or Scarlett might have drawn on contacts made while working in the security and intelligence field for the UK government, or that SCS had engaged in a form of espionage in Romania that might have undermined state institutions.

“The assertion is nonsense,” he said. “SCS investigated a case in which the client alleges that he has been subject to unjust procedures outside the rule of law.”

Ultimately, however, the judge in Adamescu’s extradition case refused to admit Scarlett and Carlile’s report as expert evidence, citing concerns about the lack of information about the identities of the company’s “unnamed but trusted Romanian agents” and the circumstances in which SCS’s operatives – identified in court as “retired intelligence officers” – collected the information.

The report illustrates how high-profile anti-corruption cases in Romania – a major political issue in the EU member state – have generated business opportunities for private investigators and outside consultants. According to Romanian media reports, Adamescu’s family-owned company previously explored hiring controversial Israeli private intelligence firm Black Cube and retained a consultancy run by former FBI director Louis Freeh.

After publication of this article, a written statement from James R. Bucknam, CEO of Freeh Group International Solutions, said: “Neither Dan Adamescu nor Alexander Adamescu have been clients of Mr Freeh or of our consultancy.” A Romanian media assertion that Louis Freeh was Alexander Adamescu’s lawyer was incorrect, Bucknam wrote in an email.

Romania’s anti-corruption directorate, the DNA, which investigated Adamescu and his father for offences including bribing judges, has brought scores of cases against senior politicians and business figures in recent years. But prosecutors have come under intense pressure from opponents, including the government, which last year curtailed the DNA’s powers and forced its chief from office.

A key section of SCS’s report, entitled “product of sensitive inquiries”, describes a series of extraordinary allegations about the conduct of the DNA and Romania’s domestic intelligence service in relation to the Adamescu investigation.

The allegations, which include falsified charges and blackmailing witnesses, are attributed to sources close to the DNA including a “well-placed official with direct knowledge and experience” of the Adamescu case.

Citing numerous “well-placed” sources, the section also includes an account of a national security meeting in the prime minister’s office in 2013 and details from diplomatic negotiations between senior Romanian and German officials in 2015.

SCS partly relied on the intelligence to support its conclusion that the DNA’s prosecutions of Adamescu and his father “bear all the hallmarks of a politically motivated campaign using the criminal law”.

Romania’s judicial authorities rejected the information attributed to SCS’s sources and objected to the admissibility of the report in extradition proceedings, expressing concern at its reliance on unnamed sources said to have provided information in “unknown circumstances to unnamed intermediaries”.

Carlile told the court that anonymity was necessary as its sources feared reprisals should their identities be revealed. Three were said to fear for their lives.Details about SCS’s high-stakes work in Romania paint a notably different picture of the company’s business activities to the description in Carlile’s disclosures in the House of Lords register of interests, which describes SCS as a company that provides “strategic advice on UK public policy, regulation, and business practice” to inward investors. Carlile told the Observer his entries in the register “comply with the registration requirements.”

In the report for Adamescu, SCS provides a clearer picture of the company’s expertise in foreign intelligence and other “sensitive matters” concerning companies and governments. The firm boasts of an “extensive cadre of trusted outside advisers” including “former intelligence chiefs from the USA and elsewhere.”

Carlile and Scarlett — lifelong friends since attending elite private school Epsom College — formed SCS in 2012. Scarlett retired as chief of MI6 in 2009 after a 38-year career in British intelligence.

SCS’s only disclosed client is the government of Qatar, but court records show the firm has worked for two high-profile Russian businessmen in London, including oligarch and former Kremlin minister Vladimir Chernukhin in connection to a dispute involving the oligarch, Oleg Deripaska. Last year, Scarlet and Carlile formed a new company, Manet Solutions, specifically to provide advice on extradition cases.

Asked about the pair’s clients, Carlile said: “We never discuss our work unless it is a matter of legitimate public record. Many other companies gather intelligence in a lawful way, as do we.”

• This article was amended on 19 February 2019 to add the comment from James R. Bucknam.