With 80-90% of all spam being routed through hijacked PCs these IP addresses were a good guide to where infected machines were located, said Professor Michel Van Eeten from the Delft University of Technology who lead the OECD-backed research.

Analysis of this huge corpus of data showed that about 50 ISPs were harbouring around half of all infected machines worldwide. Confirmation of this finding came from other non-spam sources - the 169 million IP addresses that were part of the Conficker botnet and 130 million IP addresses collected by net security watchdog SANS.

What was also clear from the research, he said, was that ISPs were not going to be able to clean up the large numbers of infected machines without some kind of central aid. In Holland, ISPs have dramatically increased their efforts but are still only cleaning up about 10% of infected machines.