Gatwickmeetandgreet.net also said it had been approved by Gatwick Police and Trading Standards.
It said it "never" overbooked customers and parked cars in a police-inspected, fenced and floodlit compound.
One reader complained cars were parked entirely in a quiet residential road.
Urban Parking, owner of the service, did not respond to the Advertising Standards Authority (ASA) questions about the complaint.
The ASA said there was no evidence to support customers' understanding that their cars would be routinely parked at the compound and would remain there for the duration of their stay.
Gatwickmeetandgreet.net's claim of having been approved by Gatwick Police and Trading Standards was misleading and unsubstantiated, the ASA ruled.
It said the advert must not appear again in its current form, saying: "We told Urban Parking to ensure their future advertising did not mislead in relation to where consumers' vehicles would be parked."
An advert for car parking at Gatwick Airport has been banned after a complaint that vehicles were being parked on local roads instead of in a secure compound.