One of the UK’s most prestigious law firms is being investigated by regulators who have spent at least two years examining it, sending investigators into its central London offices.

The Guardian has established that the Solicitors Regulation Authority (SRA) is conducting a complex investigation into Mishcon de Reya, involving forensic and anti-money laundering investigators.

The long-running investigation was launched after the SRA received separate reports between 2017 and 2018 that relayed a series of allegations about the elite London firm.

One of the reports is understood to have related to an HM Revenue and Customs investigation into transfers of Premier League football players, while the second detailed allegations about the firm’s work for clients connected to an elaborate €100m (£89m) fraud.

It is not clear whether the investigation is a result of these reports or relates to issues raised by them. Forensic investigations are typically reserved for serious cases and require firms to produce financial records for inspection, often at short notice.

A former SRA investigator said multi-year investigations were unusual but not unheard of, particularly for more complicated cases, adding: “It’s certainly not run-of-the-mill for an investigation to go on for that period of time.”

Mishcon de Reya, which has grown rapidly over the past decade, with revenues of £48m in 2009-10 jumping to £188m in 2019-20, is famous for representing Princess Diana in the 1990s and has continued to represent high-profile clients such as the businesswoman and pro-EU campaigner Gina Miller.

The ongoing investigation into Mishcon de Reya is understood not to have reached any conclusions or made any adverse findings against the firm or any of its partners or solicitors.

If the SRA makes an adverse finding against a firm, its sanctions can range from letters of advice to a referral to the solicitors disciplinary tribunal (SDT), which can strike off solicitors and issue suspensions and fines.

Between 2017 and 2018, the period for which the latest figures are available, the SRA carried out 6,000 investigations, the majority of which resulted in a finding that no rules were breached or seriously breached. More than 130 cases were referred to the SDT.

Mishcon de Reya declined to comment on the investigation. “Due to our duty of confidentiality to our clients, we cannot comment on our communications with our regulator,” the firm said. It added that the SRA had never attended unannounced, nor intervened into the firm.

The SRA’s investigation is understood to have begun after the regulator received a report about a 2017 high court judgment in a case between Newcastle United football club and HMRC. The judgment shed light on a criminal investigation into the club for alleged tax evasion, which it has previously denied.

Mishcon de Reya was not a party in the case and no findings were made against the firm. However, the judgment described HMRC’s allegations against Newcastle, including an allegation that the bulk of a £1.9m fee that the club paid to an agent who was involved in 2011 negotiations to sign Demba Ba was transferred via Mishcon de Reya to companies linked to Ba and unlicensed football agents.

The judgment recorded that in 2017 HMRC said it was investigating other player transfers and “the suspected role of Mishcons” as part of the criminal investigation known as Operation Loom. HMRC indicated that production orders “would be sought against that firm in due course”. It is not known if these orders were ever sought.

In 2018, the SRA received a second report about Mishcon de Reya when lawyers for a defendant in one of the largest private prosecutions in the UK filed a complaint against the firm.

While the full details of the complaint are unknown, it is understood to have included allegations of anti-money laundering failings and conflicts of interest relating to Mishcon de Reya’s work for one of its former clients, Luis Nobre.

Nobre, a Portuguese confidence trickster who posed as a City trader with links to the Vatican, was later found to be behind a €100m investment fraud against the Dutch shipbuilding company Allseas. He was imprisoned in 2016 for 14 years in connection with the fraud.

The defendant in the private prosecution case, Paul Sultana, was convicted in 2018 at the end of a six-week retrial for his role in the same fraud. Mishcon de Reya, which represented Allseas, prosecuted the case for it privately after the Crown Prosecution Service refused to charge Sultana.

The allegations contained in Sultana’s complaint to the SRA were originally raised in his first trial when his lawyers attempted to halt the prosecution on grounds of abuse of process and alleged conflict of interest, according to court transcripts.

The judge in the case described some of their complaints as “serious”, but dismissed conflicts of interest allegations against the firm and ruled the complaints were irrelevant to the criminal proceedings. He said the proper forum for dealing with the other concerns was the SRA.

The SRA declined to comment on the investigation. A spokesman said: “We do not normally comment on the details of our investigations. It is only if action is necessary that it becomes a matter of public record.”

A spokesman for Newcastle United declined to comment on the HMRC investigation. An HMRC spokesman said: “We do not comment on identifiable taxpayers and neither confirm nor deny ongoing investigations.”