The organisation "London Councils" - which represents the boroughs - says charges are determined by the demand for parking spaces.
But critics claim they just add to the burden on already hard-pressed Londoners.
Our Political Correspondent Simon Harris reports:
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The mayor said a post-Brexit visa system was needed to maintain the supply of skilled migrant workers for London's economy.
Peter Barnett made hundreds of trips from his home in Oxfordshire to Marylebone station without paying the full fare.
Police say he may have attacked more children and urged them to come forward.