Some travellers will pay to check in a second piece of luggage under new rules introduced by British Airways.
BA passengers face new restrictions on their luggage allowance
Passengers will be limited to a single 23kg bag - unless they travel to the US or other countries whose rules are based on pieces of luggage, not weight.
Travellers with an extra bag will pay £240 return for long-haul, £120 for European and £60 for domestic flights.
The rules will not be fully enforced until 30 September - and those who find a single bag too unwieldy are exempt.
BA said it would handle the process of applying the extra charges "sympathetically", acknowleding that many passengers are unclear about what they have to pay.
In practice, this means that people guilty of "modest infringements" of the new weight limit between now and the end of September will not be penalised.
One traveller, Martin Bunce, experienced the soft launch when he checked in two bags on a flight to Oslo this Tuesday.
"They said they would allow me on on this occasion", he told the BBC.
"But had they told us online when I booked the ticket I would have packaed one larger bag and been OK".
However, those wanting to check-in much heavier luggage will be expected to pay the full amount.
A 30% discount from the fee is available for people who pay in advance on the internet.
People such as those with disabilities, the elderly or pregnant women will not be forced to pay the excess if they cannot easily handle a single 23kg bag.
Travellers to the US, Canada, the Caribbean, Nigeria, Mexico and Brazil will still be able to check in two pieces of luggage free of charge - because those countries have a system based on pieces rather than weight.
Customers have previously been allowed to check in more than one bag as long as their luggage did not exceed weight restrictions.
In some cases the previous weight restriction was 32kg.
Passengers will be able to carry one piece of sporting equipment free of charge in addition to their baggage allowance.
BA says its new policy is simpler and will only affect a tiny percentage of its customers.
A spokeswoman said the policy was being introduced to comply with new health and safety regulations designed to protect baggage handlers.
Chief executive Willie Walsh said at the weekend he would not back down over the plans.
"A policy of charging for excess baggage has existed in the airline industry since the Wright brothers, and we're not changing that," he said.
From 30 September, the maximum weight of a single bag that can be checked in for a BA flight free of charge will be 23kg.
First and Club World class passengers will be allowed to check in three bags weighing up to 23kg each for free. Club Europe and World Traveller Plus passengers will be allowed two.