Ticket touts: 'The dark side of London 2012'

From the moment they first went on sale, to the sight this week of rows of empty seats, Olympic tickets have been one of the most controversial topics of the London games.

Yet despite a police crackdown, the BBC has found that Olympic ticket touts are still operating.

And illegal websites are also offering tickets which they have no right to sell, and which people may never receive.

The BBC's Richard Bilton reports.