German investigators have raided the Bundesbank and offices of national airline Lufthansa in what is described as the biggest euro-fraud since the currency was introduced.

Police allege at least EUR20 million worth of scrapped coins were sent to forgers in China where they were minted again and brought back to Germany.

Lufthansa flight attendants are alleged to have smuggled the counterfeit coins back into Germany and exchanged them for paper currency from accomplices in on the scam at the Bundesbank, Germany's central bank.

Private offices and residences were also raided in co-ordinated sweeps across cities and towns in the Rhine-Main area on Wednesday. Six people were arrested over the fraud.

State prosecutor Doris Muller-Scheu confirmed: "An investigation for putting counterfeit money into circulation is under way against the suspects."

The Bundesbank regularly takes out of circulation bent, scratched and dirtied euro coins to be replaced with freshly minted ones.

The process of destruction involves separating the cupro-nickel core of the euro from its outer brass ring. The scrap is sold - but in the past year it has been bought by an organised criminal gang that sent it to China where the coins were reassembled to become replicas of the originals.

When flown back to Germany every transaction to change the coins into paper was under EUR10,000. Anything more than that requires paperwork involving customs declarations.

Sleuths got on to the fraud when one flight attendant was spotted coming through Frankfurt Airport early last year pulling a trolley suitcase with great difficulty. A random search revealed she had thousands of fake EUR1 and EUR2 coins inside.

Germany's K65 organised crime unit is in charge of the probe. A spokesman said at least 25 more people are wanted in connection with the fraud.