Parking fines are set at £80 or £120 for more serious offences
Motorists who have paid to park their cars in the City of London are being fined if they forget to remove previous tickets from their windscreen.
New City of London Corporation guidance instructs wardens to issue fines "if more than one ticket is visible".
The corporation said it was vital that motorists displayed only one ticket at a time.
Parking campaigner Barrie Segal criticised the move, saying it "can only be about raising money".
Nearly 5.5 million parking tickets were issued across London between April 2008 and March 2009.
Some fines are set at £80 and more serious offences - such as parking on a single yellow line - result in a £120 charge. The fine is reduced by 50% if paid within a specified period.
In guidance on its website, the City of London Corporation explains when it will issue fines.
It states that only a single pay and display ticket must be visible, and old tickets must be removed.
The corporation said: "If more than one ticket is visible, the civil enforcement officer will issue a penalty charge notice and it is unlikely that we will withdraw it."
It declined to comment further.
Parking campaigner Barrie Segal said the policy was unjustified as drivers often forgot to remove old tickets from their windscreens.
Mr Segal said: "This sort of matter is the sort of thing which motorists say can only be about raising money.
"Why would you penalise somebody for having a valid ticket, and another one in the windscreen?"
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