Anyone who has been through an airport in the last two decades will be aware of the common security rule that bans various items, including bringing through a certain amount of liquid. Few realise why the liquid rule was introduced in the first place, and why it is still followed today.

With more people saving money by packing hand luggage only for their holidays, the rule could be broken very quickly, with toiletries pushing them over the liquid limit. At many airports, you cannot take liquids in containers larger than 100ml through security.

Some believe that the rule was brought in following the September 11 attack on the World Trade Centre in New York back in 2001. However, it didn't actually come into force until several years later.

Explaining the rule, Chelsea Dickenson (of @Holidayexpert) said: "It actually came five years later in 2006, because the Met Police foiled a plot that could have taken place. Could have been awful." Following the failed plot, Chelsea explained: "First of all, [airlines] banned anyone from taking any carry-on luggage for a few days. You had to check everything in so that it could be properly checked."

The blanket ban lasted only a few days, between August 10 and 14. Carry-on baggage was eventually reintroduced to its 'normal' over a period of time, but the liquid rule stuck for the next two decades.

The reason 100ml was chosen was to prevent terrorists from carrying liquid explosives on board, disguised as beverages or toiletries. The limit is designed to be small enough that liquid components cannot be combined to make a destructive, large-scale explosion, while being manageable for security screening.

There are plans in place to update the rules as technology advances, but most airports still strictly enforce the 100ml limit. However, the rules are set to change again.

Airports in the UK are gradually allowing passengers to carry up to two litres of liquids in hand luggage without having to remove them from their cases. However, the plans are not in place at all UK airports just yet.

For some major UK airports, the rule has already changed, but others still need to catch up with the rollout of new scanning machines. The 100ml rule has been removed at several airports, with passengers at Heathrow, Edinburgh, Belfast and Birmingham able to take up to two litres of liquids through security.

These changes are due to new CT scanners that provide more detailed images, but the roll-out of the scanners is several years behind schedule. At some airports, including Luton and Manchester, passengers still need to follow the 100ml rule - although they can keep liquids in their cabin bags.

Below is a full list of UK airports that still have the 100ml rule in place:

At most airports, there's no limit on the number of bottles you can take with you -regardless of the 100ml rule being in place or not. However, it is best to check the airport's website for the rules.