Summer holiday season is officially underway, and an estimated 63 million passengers are expected to fly out of UK airports between June and September.

Short-haul flight tickets to popular locations such as Majorca, Portugal, Italy and Croatia can often appear to be incredibly cheap, but the cost can quickly double once optional airline extras such as luggage and a seat reservation are added - especially if these are left to the last minute.

Airport Parking and Hotels (APH), estimates that these two extras alone can double if not treble the original price of a flight ticket this summer.

Despite this, they are the two most popular extras for Brits travelling abroad, beating travel insurance and airport parking.

Glasgow Times: These are the best airlines to travel on for cheap seats and extra luggage costsThese are the best airlines to travel on for cheap seats and extra luggage costs (Image: Getty/ViktorCap)

With that in mind we looked at which airlines provided the cheapest options for seat reservations and extra luggage.

Which airline is cheapest to reserve a seat on?

A seat reservation is the most popular extra, with over a quarter or recent passengers opting for this (27%).

Some groups may feel forced to pay for this extra though, as while many airlines will guarantee that one parent is sat next to their child (if aged 12 or under), there is often a charge to guarantee that the other parent and any older siblings are also sat nearby.

But how much does a typical seat reservation cost?

  • Ryanair: £2-£30 per flight
  • easyJet: £0.99-£30 per flight
  • Jet2: £2-£20 per flight
  • TUI Airways: £0.99-£19 per flight

Which airline charges the most for extra baggage?

Extra luggage was the second most popular ‘extra’ with one in six (16%) paying to take a 10kg bag with them.

One in eight (12%) added an extra-large suitcase (20kg+) to their short-haul flight ticket instead.

But how much does extra baggage typically cost?

  • Ryanair: £12-£35.99 for 10kg per flight / £17.99-£59.99 for 20kg per flight
  • easyJet: £9.49-£40 for 23kg per flight
  • Jet2: £20-£45 for 22kg per flight
  • TUI Airways: £23-£50 for 20kg per flight

Nick Caunter, Managing Director at Airport Parking and Hotels added: “Booking added extras on flights will always be a personal preference, whether adding a seat reservation or extra luggage. These findings show just how popular they are.

“It is positive to see that the majority find the extras worthwhile too, but the cost of certain extras like seat reservation can be lower if you add them to your booking nice and early.

“It is also worth shopping around for extras such as airport parking and travel insurance, it may be easiest to book through airlines, but savings could be made elsewhere.”