Ryanair has flown into a row over plans to charge £6 to check in more than four days before a flight.

Europe’s largest budget airline is to close a window for advanced online check-ins from next month, forcing passengers to find internet connections while abroad to avoid charges when they reach the airport.

The carrier said that the change was designed to benefit passengers who pay extra to reserve seats in advance by giving them more time to choose the best place on the flight.

The change comes just months after the Civil Aviation Authority confirmed it was investigating airline fees amid complaints that many charges were hidden from passengers.

Ryanair passengers can currently pay between £8 and £15 to reserve seats in advance, allowing them to check in up to a month before flying.

Those who do not pay for a seat reservation can check in a week before a flight, when they can choose where to sit. In many cases, this allows families going away for a week to check in for outward and inbound flights at the same time.

However, passengers with unreserved seating will see their advanced check-in window cut from seven to just four days from November 1.

The change means that these travellers will be forced to find internet access abroad or pay expensive roaming charges to use their mobile phone to check in. Otherwise, they will have to pay £6 to check in before the four-day window opens.

Experts warn that the change increases the chance of missing the online check-in system completely, leading to charges of £45 each if passengers have to complete the process at the airport.

The consumer website Moneysavingexpert said: “Whatever you do, don’t leave checking in until you arrive at the airport. You’ll pay £45 per person, per flight if you fail to do it online in advance — £180 for a family of four.”

The airline’s chief marketing officer Kenny Jacobs told The Times: “We’re offering those customers who wish to reserve seats more time to choose their preferred seat, by reducing the check-in window from seven to four days pre-departure for those customers who prefer a random seat.

“Customers who do not wish to reserve their seat will be able to check in between four days and two hours ahead of their departure . . . and will continue to be randomly allocated a seat, free of charge.”