Drivers are unwittingly paying up to 40% more to park as more councils switch to app-only payments and force customers to pick up the tab.

Many councils across England have removed pay and display machines and replaced them with app payments, including a number in London. Most local authorities encourage drivers to pay by app to beat queues, while apps are often the only way to pay when ticket machines are out of order. However, many car park signs fail to mention that app payments may incur an additional service fee.

Parking apps save councils money by removing the cost of installing and maintaining ticket machines, but while a few pay the app providers directly for their services, many leave drivers to foot some or all of the bill through "convenience fees" or "service fees".

These typically range from 10p to 30p, but they are not included in the advertised parking tariffs in car parks, even when an app is the only payment option, and they are often not strongly flagged within the apps.

Of the parking apps tested by the Guardian, none clearly displayed the service fee until the payment stage, and even then, some required customers to click on an information symbol to find out why the price had increased.

These surcharges can push up the cost of an hour's parking slot dramatically. In Harrogate, North Yorkshire, drivers who park in the town centre and pay by app are hit with a mandatory 30p service charge – a 38% rise on an 80p tariff.

Meanwhile, the price of two hours at the council's Christleton Road car park in Chester soars by 40% when the 20p convenience fee is added to the 50p tariff.

Extra fees are added if drivers agree to receive booking confirmation and expiry alerts. They are typically between 10 and 30p per text.

According to the Advertising Standards Authority, quoted prices must include all mandatory fees for a contract to be valid. Drivers enter a contract with a council as soon as they accept the car park's terms and conditions and park their vehicle.

However, few realise that they enter a separate contract with the app provider if they choose to pay online. App fees, therefore, may not legally have to be displayed on parking council signs.

Because service fees are agreed between local authorities and individual app providers, and since they can vary depending on location, drivers have no way of predicting what they will be charged.

The Guardian tested the apps by booking an hour's parking at car parks around the country.

RingGo's convenience fee added an extra 20p to the £2 tariff at Kings Yard in Lancaster. Drivers have to click on a small "see prices" link above the prominent "Next" button when they have chosen their location to see the charge. It is mentioned again on the payment page, but only when the "i" symbol beside the final price is clicked.

PayByPhone charged a 15p service fee on top of the £5.10 parking tariff at Dudley Street car park in Birmingham. The existence of the charge was not explained until the payment page and was hidden behind the Apple Pay button. Customers have to scroll down to the end of the page to see it.

Customers using JustPark have to click on a tiny pale grey "i" symbol beside the final price at the end of the booking process to discover its transaction fee. This added 12p to a £3.50 parking charge at Charmouth Road car park in Lyme Regis, Dorset.

MiPermit listed its 10p convenience fee above the final price at the end of the booking. This was levied on top of the £1.90 parking charge in Littlehampton, West Sussex.

RingGo said the fee funded the service it provided and that it was up to the councils it worked with whether or not to display it on car park signs. "In line with existing regulations, wherever pricing information is shown in the RingGo app, we also display any additional fees," said Adam Dolphin at its parent company Arrive.

JustPark said mandatory fees were included in the price quoted during payment. "Nothing is added at the payment stage," said a spokesperson. "The 'i' icon simply shows how that all-in price is made up, for drivers who want the detail."

PayByPhone said its pricing was transparent. "The pricing elements related to parking, SMS options and service fees are presented before payment confirmation," said a spokesperson. "Applicable pricing information – whether fees, parking rates or options – is available on the website and in the app."

MiPermit was contacted for a comment.

The Local Government Association, which represents councils, said it was up to app providers to ensure their fees were clear and upfront.

The Department for Transport said parking tariffs and surcharges were a matter for councils. "Drivers should be able to see clearly what they will be charged before they pay for parking," a spokesperson said.

A spokesperson for the AA said: "When a driver walks to the signs in the car park and sees £1 per hour, that's what they should be charged, not £1 plus a 25p service charge."