British officials have warned travellers not to bring marijuana from Thailand, where the drug has been decriminalised, after a spike in arrests at UK airports.

The British embassy in Bangkok warned in social media posts Wednesday that the country's National Crime Agency (NCA) was on the lookout for people smuggling cannabis in their luggage.

The warning came as the NCA said it had seized 15 tonnes of the cannabis already this year and arrested 378 smugglers -- half of them over drugs originating in Thailand.

Thailand decriminalised marijuana in 2022 and "dispensaries" selling the drug either loose or as pre-rolled joints are now a common sight in Bangkok and other major tourist centres.

But it remains illegal in Britain, and smugglers can face up to 14 years in jail.

"The NCA is keeping a close watch on the illegal import of cannabis into UK airports," the British embassy said in posts on X and Facebook.

"The UK is working with other partners to stop attempts to smuggle illegal products and will put those committed to justice."

British officials say smuggling gangs are recruiting couriers by telling them they will only be fined if they are caught.

Earlier this month, 11 people travelling from Thailand were arrested at Birmingham airport after 510 kilos of cannabis were found inside 28 suitcases, the NCA said.

The agency said it has arrested 184 people over cannabis smuggled from Thailand this year, as well as 75 from Canada and 47 from the United States.

In 2023, there were only 136 arrests and five tonnes of cannabis seized.