Most of Avis’s rivals, including Europcar and Sixt, do spell out the need for the cover and its cost at the time of booking.

This gives travellers the option to shop around and consider stand-alone car car hire excess waiver policies which can cost as little as £3 per day.

Avis’s own policies are far more costly. Booking a four-door Seat Leon from Malaga Airport for a week in mid-April costs £143 with Avis, for example. The excess payable is €1,400 (£1,020). The add-on insurance required to reduce the excess to zero – not disclosed to those booking online – is €15 (£11) per day.

That would boost the total car hire cost by over 50pc to £220.

Telegraph Money first publicised Avis’s systematic failure to reveal these add-on costs in August last year. The giant car hire firm then acknowledged the problem and promised “a solution”.

But nothing has changed.

Last week, Avis refused to explain why these costs remained hidden from customers at the time of booking. When Telegrpah Money asked whether this refusal was a deliberate ploy to trap customers into paying higher rates at Avis’s desks, it declined to reply.

In an emailed statement it admitted “certain covers and waivers for overseas rentals are not available when booking online” and said “customers will be given the opportunity to find out more about additional waivers and, if desired, purchase them at the time of vehicle pickup.”

It added it was “in the process of updating the end-to-end booking journey.”

In the meanwhile, when it comes to planning their own journeys, Telegraph Moneyreaders should choose another firm where costs are more transparent – or always buy excess waiver policies from third-party insurers before they travel.

by Richard Dyson