The Bank of England is to begin monitoring Lloyd's of London after it emerged that a whistleblowing hotline at the insurance market was out of service for almost a year and a half.
It meant about 1,000 staff had no way to anonymously report issues such as sexual harassment or bullying.
Lloyd's has been trying to clean up its image after a wave of female staff complained of being harassed at work.
The company said it was "disappointed" by the failure in its controls.
About 50,000 people work at the Lloyd's of London insurance market, where brokers and insurers meet to do business.
In February, the firm discovered its "Speaking Up" hotline - which was only open to directly-employed staff - had been out of action for 16 months.
Lloyd's alerted the Bank of England's Prudential Regulation Authority (PRA) to the fact at the time. It told the body, which oversees City firms, that it had forgotten to renew its contract with the external company that managed the line.
Staff could have used an email address and app to report issues, but not anonymously, the PRA said.
The City firm has now agreed to improve its reporting processes and will face "enhanced scrutiny", the PRA added.
Lloyd's has been trying change its male-dominated culture following a highly critical article in Bloomberg Businessweek magazine in March. It reported female workers had faced inappropriate comments as well as physical attacks from male colleagues.
A survey later commissioned by Lloyd's indicated that 8% of workers had witnessed sexual harassment over the previous 12 months.
To tackle the problem, the 330-year-old market has:
- put curbs on daytime drinking
- introduced life bans for anyone found to have behaved inappropriately
- put posters up in the toilets of pubs near its offices in the City of London
- urged staff to report sexual harassment.
Earlier this month, it also warned staff to behave during the Christmas party season.
Commenting on the hotline, a spokesman said: "We are extremely disappointed by this failure in our internal controls, which serves to remind us all about the need for constant vigilance when it comes to these essential services.
"Lloyd's employees can feel confident that we now have all the right mechanisms in place for them to report any wrongdoing, and that these systems are regularly monitored."