The failure to designate reinsurers has angered insurance companies, which argue reinsurers are more important to the financial system. In a separate move that underscores concern about the increased scrutiny brought by designation, MetLife has sued the US government to try to escape being deemed systemically important by Washington.
Insurers deemed systemically important on a global level may need to hold more capital to cover unexpected losses and could face a requirement to draw up "living wills" to make them easier to wind down in a crisis. Yet regulators have yet to quantify the scale of the HLA requirements and the consequences of designation remain unclear.
The Basel-based FSB was expected to make the reinsurance list public last year. But in November, following consultation with national authorities, it postponed the decision "pending further development of the methodology".
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In a sign that disagreement between global regulators may be holding up the process, it has emerged that Bank of England officials wrote to US authorities in October seeking clarity about Berkshire's absence from a provisional list.
The Bank confirmed the existence of the letter in response to a request under UK Freedom of Information legislation from the trade publication Risk.net. However, it declined to disclose the letter's contents.
Insurance has been vital to Berkshire's transformation into the largest conglomerate in the US in the 50 years since Mr Buffett took control. Premiums provide capital that he can invest in stocks and other businesses.
It is Berkshire's most significant business. Underwriting and investment profits accounted for 27 per cent of its net earnings in 2014.
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AM Best, the rating agency, ranked Berkshire sixth among global reinsurers as measured by gross premiums written in 2013 — behind Munich Re, Swiss Re, Hannover Re, Lloyd's of London and Scor.
The size of Berkshire's balance sheet, however, means it has considerable untapped underwriting potential. Mr Buffett has pulled back from underwriting large reinsurance risks in recent years, because of his concern that premiums have fallen too low.
Size per se is not regulators' main criteria in determining which insurers are considered systemically important. Watchdogs have focused instead on the "non traditional" and "non insurance" business that the institutions conduct.
In the parallel US process, Berkshire crosses thresholds that would allow it to be designated, with more than $50 billion in assets and more than $3.5bn of derivatives liabilities. However, the final decision is made by a council of US regulators which has not yet opted to subject Mr Buffett's company to increased oversight.
The debate comes as several insurance executives are complaining about mounting regulatory scrutiny of their industry, saying they have been unfairly lumped in with banks since the financial crisis. Still, some consultants believe that the global capital requirements for European insurers could ultimately prove less onerous than the EU's forthcoming Solvency II regime.
Berkshire declined to comment. The company cites "regulatory changes" among risks to its business in its annual report, although it has not addressed the potential consequences of designation as systemically important.
The US Treasury declined to comment, as did the Federal Reserve, which is also a member of the Financial Stability Board.