Friends of Bushy Park said it believed that Margaret Hodge will push through the charges in the two Royal Parks-run green spaces, following a letter sent to chairman Pieter Morpurgo.
A consultation on the proposed charges was carried out earlier this year and showed 84 per cent of people opposed the plans. The Government will make a decision on the proposals in the new year.
Morpurgo said: "We do not understand why Hodge wants to impose her policy of charging people to use the park while letting through-traffic use it as a rat run scot free.
"Car park charges will ruin the experience of the park for many people and won't even raise much money."
Keep Bushy Park Free Alliance representative Barry Rideout added: "This is just the same as when they imposed entry charges on museums. Visitor numbers went through the floor.
"While that mistake was eventually rectified, it seems they're about to repeat it here."
In the letter sent by Hodge to Morpurgo, she stated: "I know that a great many people are opposed to the inroduction of parking charges in Bushy Park and it is understandable that visitors are reluctant to pay for a service that was previously free.
"I do not accept your assertion that the level of charge will deter people from visiting. It may have the effect of encouraging visitors to use modes of transport other than the car, which has to be good for the park.
"I should also add that the proposed charge of £0.50 an hourto a maximum of £2 is not excessive and Bushy and Richmond Parks are the only Royal Parks with parking provision that do not charge."
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