On top of this the firms showed weaknesses in the underwriting of commercial loans and the review found instances of weak financial analysis, limited evidence of challenge and high levels of lending outside of policy.
Ms Beaman wrote: "[Firms] need to ensure they are prepared to react quickly in the event that credit conditions change."
Further investigations into fast growing firms’ funding and lending analysis re-enforced earlier findings that many challenger banks were reliant on easy access to retail funding and were in "pursuit of aggressive balance sheet growth targets".
Ms Beaman concluded that the findings warranted a general reminder of the importance for all firms to ensure their governance and risk management remained aligned with their business model risk profile and appetite.
The BoE also pointed out that the review provided reassurance about the overall resilience of the sector and that "some weaknesses is not unexpected" given the early stage of development of many fast growing firms.
Commenting on the letter, UK Finance, the bank trade body, said these types of lender provided a valuable source of finance for small businesses and consumers while "helping drive competition and innovation in the marketplace".
The spokesperson added: "The regulator’s feedback is helpful in supporting an improvement in the resilience of these kind of businesses as they grow.
"UK Finance will continue to support its members and liaise with the regulator to ensure this important part of the market continues to work effectively."
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