If excess waiver insurance is NOT purchased by a hire car customer they could be liable for up to Â£1500 if the car gets damaged or stolen.
Even a small scratch on a windscreen can cost hundreds to fix.
But many rental companies do not make this clear.
Research by insurer iCarhireinsurance.com found that the five leading car hire companies surveyed charged an average of Â£17 a day for excess waiver car hire insurance, with the highest being an eye watering Â£23 a day with Europcar.
However, policies can be bought online for Â£2.99 a day.
Ernesto Suarez, chief exec of iCarhireinsurance.com said: "Excess car hire insurance is sprung on people when they arrive at the car rental desk to pick up their car.
"Tired after their journey they are then held to ransom to take out excess insurance with the fear otherwise of being liable for huge penalties, when all they want to do is pick up the keys and start their holiday."
Read our original car hire excess report here.