One of London's free newspapers has hired a private investigator to snoop on its rival.
Both proprietors accuse the other of dirty tactics
London Lite claimed vendors handing out copies of The London Paper were dumping thousands of copies to increase circulation figures.
But The London Paper, owned by Rupert Murdoch, said it had photographs showing its rival doing the same thing.
Both papers face a distribution ban from Westminster Council unless they provide more cash for recycling.
Figures released by the Audit Bureau of Circulation in December showed that The London Paper beats its Associated Newspapers rival.
Pattern of dumping
It had 425,000 copies distributed, 30,000 copies more than the London Lite during the month of November.
But Associated Newspapers claim Rupert Murdoch's freesheet is distorting the market by throwing away thousands of copies of its own paper.
In a statement, Associated Newspapers said: "Our investigation shows a repeating pattern of dumping by The London Paper vendors at different locations in central London.
"They are desperate and oversupplying the market."
A statement from The London Paper said: "We have photographic evidence of Associated dumping copies of The Lite.
"In March, photos were sent to senior executives at Associated highlighting the extent of this dumping."