Sometimes travel plans don't go as intended, and when booking connecting flights it's not unheard of for passengers to occasionally miss a part of the journey. 

But doing so can cost travellers a lot of money because of a lesser-known 'no show' clause – and travel experts have revealed to the Daily Mail why this is the case.

In one case, three holidaymakers skipped their flight from Glasgow to Heathrow Airport and instead got the train down.

However, when they tried to board their flight from the UK's busiest airport to Mexico City, they were turned away and told that, because they had not taken the first part of the journey, their tickets were now invalid.

In the end, they coughed up £9,000 for new return tickets and maxed out their credit cards.

British Airways told The Guardian that passengers are asked to confirm they have read the conditions of carriage when they book, and pointed towards the FAQs on the website.

The FAQs state: 'If you don't tell us in advance about any changes you need to make to your journey and choose not to travel on one of your flights, you'll be considered a 'no-show' and all remaining flights in your journey will be cancelled.

'If you need to change or cancel all or part of your journey and your fare type allows it, we can recalculate the new fare for your proposed change, or any refund you might be eligible for. Visit Manage My Booking to make changes or call us for more help.

'If you booked with a travel agent or a third party, please speak to them about making changes to your booking.'

Meanwhile, the General Conditions of Carriage read: 'Your ticket is valid only for the transportation shown on it from the place of departure through any agreed stopping places to the final place of destination.'

The website outlines how the ticket won't be valid 'if you do not use all the coupons in the sequence provided in the ticket'.

If travel plans change, the airline asks passengers to contact them beforehand.

Nick Parkhouse, a travel insurance expert from Go Compare, explained how skipping the first leg of the journey is 'one of the biggest mistakes passengers can make'.

'A traveller might do this, for example, if they think it's easier to skip the first leg and just make their way to the airport from which the second leg departs,' Parkhouse says.

'Most airlines operate under a 'no-show' clause, which means if you fail to take the first flight in your itinerary, the airline can automatically cancel all remaining flights on the booking.'

He admitted that most 'are completely unaware' of the rule and only discover it when 'they're faced with huge last-minute rebooking costs at the airport'. 

'People often see flights from regional airports as inconvenient and think they can just start their trip from a major hub instead, but airline ticketing systems don't work that way,' Parkhouse adds. 

Meanwhile, Tim Knighton, a travel expert at Compare the Market, said the policy is often in place to prevent tactics like 'hidden city ticketing'.

This is when 'passengers intentionally skip part of a journey to take advantage of cheaper fares', he said. 

'However, travellers can still be caught out for legitimate reasons such as delays, weather disruption or changes to travel plans,' Knighton adds. 

'If you think you may miss part of your itinerary, it's important to contact the airline as soon as possible before the flight departs.

'In some cases, the airline may be able to recalculate the fare or amend the booking, depending on the ticket type and terms.'

But, travel insurance is unlikely to cover any issues if a passenger purposely misses part of their journey. 

Parkhouse reveals insurers could view the situation as a 'deliberate decision by the passenger and a contractual issue rather than an unexpected disruption'.

Echoing Knighton, he added: 'If plans change, passengers should always speak to the airline before missing any part of a connecting journey.'