Some countries that have pioneered “flat” taxes could be forced to abandon them, according to research published by the International Monetary Fund, which said the advantages of imposing a single tax rate had been exaggerated.

“While there will no doubt be new members of the flat tax community, in some respects the more interesting question is whether there will be any defections,” said the working paper. It conceded flat tax reforms had become “something of a craze” in recent years.