Eurozone governments could make a windfall gain of about E6bn if their citizens fail to exchange all of their national currencies for euro notes and coins, according to a new report.

A study by two academics forecasts that the windfall profit will emerge because large numbers of people will forget or not bother to return old currency to banks before the deadline for conversion to euros.

A large part of unreturned currency are likely to be coins forgotten in piggy banks or not changed because of their low value. The deadlines for converting national currencies to euros differ in the 12 countries, ranging from the end of next year for Portuguese escudo coins to 2032 for Dutch guilder notes.

The study, by Professor Arnd Huchzermeier of Koblenz University and Ludo Van der Heyden of France's Insead business school, states: "The great winners are indeed the national Treasuries. [They will] see at least 1 per cent of the currency officially in circulation not returned for exchange into the new currency."

The possibility of a E6bn windfall is likely to lead to calls by retailers, banks and vending machine operators to be compensated for the costs of the changeover to euro notes and coins.

The European Central Bank, co-ordinating the changeover, has rejected their demands, arguing that every group, from national governments to consumers, should bear part of the costs of the euro launch.

The academics' report states: "This windfall profit might contribute to reimbursing, at least partially, those who have to make real investments in their operations to ease the launch".

The ECB declined to comment on Wednesday but officials privately argued that it was difficult to calculate the exact amount of unreturned money.