Up to a quarter of airline passengers are being forced to place their carry-on luggage in the hold, potentially putting uninsured valuables at risk, despite their bags meeting size and weight restrictions.

According to a Which? Travel study, Ryanair has the worst record of eight major airlines, with 26 per cent of fliers reporting that their cabin bags were taken from them because of a lack of space in overhead lockers.

For easyJet passengers, the figure falls to 15 per cent, while 11 per cent of Aer Lingus and seven per cent of Monarch passengers can expect to find their carry-on bags put in the hold.

It means those passengers face a wait in the airport for their luggage to arrive at the carousel, potentially meaning missed connections. Furthermore, valuables that are placed in the hold are unlikely to be covered by travel insurance, while many airlines state in the terms and conditions that they will not pay out for items that go missing. Which? Travel said its readers have reported jewellery, cash and tablets going missing from cabin bags that were unexpectedly put in the hold - none of whom were compensated for their loss.

Passengers face a wait in the airport for their luggage to arrive at the carousel, potentially meaning missed connections Credit: 2015 Getty Images/Pablo Blazquez Dominguez

It added: “Valuables are also unlikely to be covered by travel insurance. Aviva, Axa, Churchill, Direct Line and LV all exclude cover for any valuables placed in the hold, regardless of whether the passenger had intended to keep their bag with them on the flight.”

In a bid to minimise spending on fuel and check-in staff, airlines have been urging travellers to travel without hold luggage for years. But it means space in overhead bins is increasingly scarce - particularly on budget carriers. Passengers, usually those with larger wheeled carry-on cases or those among the last to board, are often told without notice, or sufficient time to switch their valuables into another bag, that their luggage cannot go in the cabin. It means many are separated from expensive or fragile items, or in-flight essentials such as medication.

The problem is particularly acute on Ryanair, which began permitting passengers to bring a second smaller carry-on bag in October 2013, but recently threatened to scrap the perk because passengers are attempting to pack “half the contents of their home” into them.

According to Which? Travel, Ryanair's 737 aircraft, which seats 189 passengers, can accommodate just 90 cases. Ryanair denied this, saying there is room for 190 standard bags. "If each passenger brings up to two permitted carry-on bags then some of these 380 bags will be put in the hold free of charge," a spokesman said.

But it isn’t just low-cost airlines that are struggling to accommodate travellers’ cabin luggage. Around one in 20 British Airways passengers also face having their bags moved to the hold.