This weekend – one of the busiest periods on the flying calendar – is likely to see thousands of airline seats standing empty, going by the usual numbers of last-minute cancellations or would-be travellers turning up too late for their flights.

For airlines, these empty seats can translate into windfalls running into millions of pounds as they pocket government taxes and other passenger charges. But passengers who miss a flight, for whatever reason, are being advised to reclaim taxes and surcharges on otherwise non-refundable tickets.

The recent doubling of air passenger duty (APD) – or UK departure tax – to £40 for long-haul economy flights has made it all the more worthwhile to claim refunds for these and other extras on unused tickets. Such refunds on cancelled or missed flights can be worth as much as £150 on a transatlantic flight costing as little as £200.

“A lot of people do not think to reclaim,” says Simon Evans, chief executive of the Air Transport Users Council (AUC). “But thousands have successfully claimed and most carriers will refund (at least) government taxes.”

The AUC says there is no law requiring airlines to rebate any so-called TFCs – taxes, fees and charges – on non-refundable tickets. But in practice even the low-cost carriers such as Ryanair and Easyjet say they will refund both air passenger duty and overseas government taxes.

And some traditional airlines will go even further. BA will refund “anything that’s separated out of the fare.” This includes fuel surcharges and passenger service as well as insurance and security charges. “BA has decided that passengers should only have to pay these charges if they actually travel,” says a spokesperson.

Even so, airlines do not advertise or automatically make these refunds, so non-fliers do need to put in a claim. And the real catch is that, in most cases, an administrative charge is also levied, which at worst wipes out any refund or makes reclaiming barely worth the bother. “Often the refundable extras will be £20 but the administrative charge will also be £20,” says Evans at the AUC.

For example Ryanair refunds passenger duty and the equivalent overseas departure tax but charges a £15 administration fee. On a return domestic flight, with passenger duty of £10 each way, that refund would therefore be worth just £5. To make matters worse, Ryanair refund seekers also have to send their claims by post to its head office in Dublin.

By contrast Easyjet makes no charge for refunding the same government taxes, and on a flight to Morocco this would be worth £40. “You are talking a lot of money now – particularly for families – and a large percentage of the ticket price since APD (departure tax) has doubled,” says an Easyjet spokeswoman.

The biggest refunds are likely to be had on unused long-haul tickets. BA says that at present as much as £157 of TFCs would be refundable on a return from London to Los Angeles, including £40 passenger duty, a £70 fuel surcharge (£35 each way) and a £5 insurance surcharge. The administration fee on an e-ticket is £15, or £30 for a telephone or postal rebate – costs that would still return more than £100 to someone who did not travel.

Virgin Atlantic said it would not charge any administration fee and refunds could also be worth more than £100 on a transatlantic or other long-haul flight.

But there can be other catches to getting refunds. If a ticket is booked through a travel agent rather than direct then refunds may have to be applied for through that agent – which at worst could mean two administration fees. Airlines may also refuse to pay any refund on return tickets where one part of the journey is taken.

The Consumers Association suggests that high administration fees should be challenged by disappointed refund seekers.

If you paid for your flight by credit card (so long as the purchase price was £100 or more), the card firm may be able to offer some help in getting money back, including paying compensation itself.

Some travel agents also point out that regardless of the ability to reclaim TFCs, air travellers should not just assume that nothing can be done with a non-refundable ticket. Their advice is to contact the travel agent concerned (or airline, if booked directly) as soon as it is known a ticket will not be used. Ryanair and Easyjet allow passengers to change their flights for fees of £25 and £17.50 respectively, which may make more financial sense than simply claiming back the tax.

Alternatively, travel insurance policies could offer financial compensation for unused tickets in some circumstances. Travel policies may cover cancellation due to the illness or death of a close relative or one of your party, as well as your own illness, redundancy or a domestic disaster. If you miss your flight due to significant transport problems getting to the airport, you may also be covered.