The security breach was discovered in May, when Russian web site SecurityLab.ru reported that it had received some of Cisco's source code from hackers.
Reports at the time suggested that as much as 800MB of code may have been stolen, although Cisco made no comment on this, other than to tell internetnews.com that, "Based on preliminary data, we don't believe any confidential customer information or financial systems were affected".
Scotland Yard spokeswoman Julie Prinsep confirmed on Friday that the Metropolitan Police Computer Crime Unit had arrested an unnamed man on 3rd September under suspicion of violating the UK's anti-hacking law, the Computer Misuse Act. He has now been released on bail.