SHANGHAI: The economic downturn might be knocking the wind out of many businesses, but one is sailing full steam ahead - the sale of illegal invoices.

Police say the desire by small- and medium-sized businesses to avoid paying taxes is the main impetus behind the growth of fake invoices.

In the past five months police have uncovered 90 cases in the city with more than 3.5 million invoices seized, the municipal public security bureau said.