Some European financial institutions should have taken bigger losses on their Greek government bond holdings in recent results announcements, says the organisation that sets their accounting rules.

In a letter to the European Securities and Markets Authority, the European Union’s market regulator, the International Accounting Standards Board criticised the inconsistent way in which banks and insurers have been writing down the value of their Greek sovereign debt. “This is a matter of great concern to us,” Hans Hoogervorst, IASB chairman, said in the letter, a copy of which was seen by the Financial Times.

People familiar with the IASB’s letter said the intervention was unprecedented and reflected the board’s belief that some European companies – particularly in France – had not been making big enough provisions for Greek sovereign debt losses.