Airline passengers across the UK are increasingly finding that the seat they booked, or even paid extra for, disappears at the last minute.

This practice, known in the industry as 'aircraft swaps' or 'equipment changes,' is becoming routine as airlines juggle fleets to cut costs, cover technical issues, or respond to delays.

Latest analysis has found that this airline procedure, swapping an aircraft just before departure, affects nearly 140,000 flights every year in the UK. And for passengers that can often mean losing a reserved seat, being downgraded, or facing overbooking when a smaller plane replaces the original.

Fleet shortages, staff strikes, and ongoing weather disruptions make last-minute swaps more likely, says AirAdvisor, an air passenger rights company.

And while you would be forgiven for thinking that your ticket guarantees a specific plane or seat it, in fact, only guarantees travel in a given class between two points. That legal grey area leaves thousands unsure of their actual rights, reports the Mirror.

AirAdvisor's analysis found between 1 per cent and 5 per cent of flights face aircraft changes within 24-48 hours before departure, rising to more than 5% at peak periods on legacy carriers like British Airways and Lufthansa.

Low-cost airlines like Ryanair and easyJet are less impacted due to their standardised fleets, but the risk increases across the board during summer demand spikes. It's vital to know your rights if you arrive for a flight only to discover that the seat you've reserved or paid for is no longer available.

These rights differ based on the circumstances.

Downgrading: If you're demoted to a lower class, UK261 regulations entitle you to a refund of 30-75 per cent of your ticket price, depending on the length of your flight.

Seat loss: If you've paid extra for a specific seat (such as an exit row, window, or aisle), you can claim a refund of that surcharge if the seat is no longer available. However, if you didn't pay extra, airlines can reassign you within the same class without offering compensation.

Overbooking due to smaller aircraft: Passengers who are denied boarding are entitled to compensation ranging from €250-€600 (£216 - £519), plus rerouting or a refund.

Unwanted aircraft type: If your flight is reassigned to a model you're not comfortable flying in (such as the Boeing 737 MAX), airlines aren't legally required to change your booking. However, some might rebook you as a goodwill gesture if you act quickly.

Currently, there's no law compelling airlines to inform passengers of seat or aircraft changes, except when they're downgraded to a lower class. This legal ambiguity can cause issues, particularly when passengers have forked out extra for seat selection or comfort.

Consequently, these swaps often occur without any prior warning, leaving passengers blindsided.

Some consumer groups have argued that airlines should alert passengers of any changes, giving travellers the right to accept or reject new seating arrangements, and offering automatic refunds when services aren't delivered as promised.

However, the ongoing review of UK261 regulations, which is under discussion in Brussels, has sparked some worry. Instead of strengthening passenger protections, certain proposals could actually dilute airlines' obligation to compensate travellers when flights are delayed or cancelled.

Meanwhile, the issue of aircraft substitutions - which are becoming increasingly common - remains largely ignored in the discussions.

For British travellers, this means a higher likelihood of discovering at the departure gate that their seat has disappeared or that their flight has fewer spaces than expected.