HSBC bank has reportedly asked more than 40 diplomatic missions to close their accounts as part of a programme to reduce business risks.
The Vatican's ambassadorial office in Britain, the Apostolic Nunciature, is among those said to be affected.
The head of the UK's Consular Corps told the Mail on Sunday the decision has created "havoc".
The Foreign Office has been in touch with HSBC, stepping in to help diplomats open other bank accounts.
HSBC said embassies were subject to the same assessments as its other business customers. They need to satisfy five criteria - international connectivity, economic development, profitability, cost efficiency and liquidity.
A spokesman said: "HSBC has been applying a rolling programme of "five filter" assessments to all its businesses since May 2011, and our services for embassies are no exception.
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