A top UK regulator has launched an investigation into how it handled a whistleblower – who highlighted alleged ties between the late sex offender Jeffrey Epstein and the US commerce secretary, Howard Lutnick – after he took his own life.

The British banker Simon Andriesz died late last month aged 57, the campaign group Transparency Task Force, of which he was a member, confirmed over the weekend.

Andriesz had formerly worked as a managing director at BGC Group, the financial brokerage where Lutnick had been chief executive. He had accused the Financial Conduct Authority (FCA) of failing to hold his former employer to account for alleged wrongdoing, and had been wrongly told he did not qualify for whistleblower protection.

The banker had spent years raising concerns about his former employer: first in regards to accounting irregularities – which he raised internally at BGC in 2016, a year before he was sacked – and then in 2020-21, when he reportedly told the FBI that Lutnick had undeclared business ties with the late sex offender Jeffrey Epstein. Lutnick has since said he had no “personal or professional relationship” with Epstein.

Andriesz, who had been living in Cornwall, later unearthed an email chain in the Epstein files that suggested Lutnick and Epstein had discussed the prospects of a startup business they were both involved in, in 2018. Epstein died in jail in August 2019 while awaiting trial on sex trafficking charges.

However, Andriesz told the BBC’s File on 4 Investigates this summer that authorities in the US and UK had failed to hold BGC and its parent company, Cantor Fitzgerald, properly to account, or protect him from potential retaliation. He also later accused the FCA of mistreating whistleblowers, and raised concerns about the toll that whistleblowing had taken on his family, career and health.

The FCA had also given Andriesz misleading information, saying he would not qualify for whistleblower protection because his identity had become known. The regulator apologised to him and said that further guidance and training would be provided to its whistleblowing team.

The FCA confirmed it had now tasked a new board member to investigate how staff handled Andriesz’s concerns. “We are very sorry to hear of Mr Andriesz’s death,” an FCA spokesperson said. “Our thoughts are with his family and friends. Since expressing our condolences, we’d asked Lea Paterson, who has just joined our board as a non-executive director, to review how we interacted with Mr Andriesz to learn any lessons for the future.”

However, the all-party parliamentary group (APPG) on investment fraud and fairer financial services is concerned that the FCA is being allowed to “mark its own homework” and called on the regulator to pay an independent party to “forensically and objectively evaluate its handling of whistleblowers”.

The APPG’s chair, the Labour MP John McDonnell, said: “It is vital that this time a truly independent and non-conflicted organisation carries out the work needed. If the FCA refuses to respond positively, the APPG will consider all alternatives, including conducting its own call for evidence from whistleblowers who have interacted with the regulator.”

Lutnick has not been accused of any wrongdoing in relation to Epstein, and has previously said: “I unequivocally condemn the conduct attributed to Jeffrey Epstein and everyone who participated in his illegal activities. The survivors of his crimes deserve our respect and support.”

A BGC spokesperson said: “BGC has long disputed Mr Andriesz’s allegations but does not intend to relitigate these matters at this sad time. BGC is sorry to hear this tragic news and expresses our condolences to Mr Andriesz’s family.”