Car hire booking sites are failing to disclose the cost of 'extras' during the booking process leaving travellers in the dark about how much they may have to pay when they arrive to pick up their car.

Extras may include the cost for an extra driver, a child's car seat, an excess /waiver policy and a sat nav.

In a study of five global car hire companies, Budget, Hertz, Europcar, Sixt and Avis, iCarhireinsurance.com, a leading provider of car hire excess insurance, found that often only half of the extra costs would be highlighted in the booking process.

With the average costs of extras across Europe totalling £277*, around £52 for a child's car seat, a sat nav costing £68, car hire excess insurance costing £111 and an extra driver £46, it’s essential that travellers have the facts of what they will be paying to hand, particularly for car hire excess insurance which can cost more than the car hire.

* The average car hire costs in five European destinations, Barcelona, Larnaca, Milan, Nice and Tenerife, to hire a compact car for a week, from 26 March to 2 April 2016. Car hire companies of Sixt, Avis, Budget, Europcar and Hertz were researched.

The study found that while travellers could usually reserve extras including a child’s car seat and a sat nav, it was not always possible to pay for them in advance, nor indeed to find out the cost of the item.

While Budget was transparent in the cost of its extras, it was not possible to pay for any of them online meaning that travellers would need to remember to budget for these additions when picking up their hire car.

Avis, meanwhile, only quotes 50% of the extras’ costs online, and then only allowed travellers to pay in advance for half of them in Barcelona; a third of them in Tenerife, Milan and Nice; and only 17% in Larnaca.

Europcar was the most consistent rental company in the study, always showing the prices of 83% of the extras searched for and allowing travellers to book and pay for these online in advance of travelling.

Sixt and Hertz only gave all the prices online for extras in three destinations overall, yet it was still not possible to book and pay for all of them in advance.

Transparency when booking hire cars continues to be a big problem, said Ernesto Suarez, CEO and founder of iCarhireinsurance.com. It’s very difficult for people to plan exactly how much they will need to pay for the hire car overall if they cannot see all the costs in advance.

He continues, Last year six of the leading car hire companies in Europe made promises to improve their treatment of customers, including their transparency of prices, following an investigation by the Competition and Markets Authority (CMA). Almost a year on it is disappointing to see that transparency is still an issue. We support the CMA's steps to make car hire companies more transparent.