Britons can fall foul of foreign laws simply by feeding pigeons or chewing gum on trains, the Foreign Office (FO) has warned.
Donning military style camouflage clothing or slipping into a bikini can lead to fines in some areas, while eating near churches is forbidden in the Italian city of Florence.
The warnings follow a recent FO report stating that 27% of cases of Britons requiring consular assistance abroad were for arrests or detentions.
The FO list of unusual laws and customs includes Venice, where feeding pigeons is against the law; Japan, where it is illegal to take some commonly available nasal sprays containing pseudoephedrine into the country, and Barcelona, where it is against the law to wear a bikini, swimming trunks or to go bare-chested away from the beach front area of the city.
Others include Singapore, where chewing gum is strictly prohibited on the Mass Rapid Transit system; Barbados, where it is an offence for anyone, including children, to dress in camouflage clothing; Nigeria, where it is illegal to take mineral water into the country; Fiji, where sunbathing topless is prohibited, and the Maldives, where public observance of religions other than Islam is prohibited for non-Maldivians and visitors.
The FO's consular services director Charles Hay said: "Every year British nationals find themselves on the wrong side of the law unexpectedly, resulting in fines or in some cases arrests or even jail sentences.
"It is important to remember that laws and customs can vary greatly from country to country and what may be perfectly legal in the UK could be subject to a fine or even a jail sentence in another."
He continued: "Consular staff often find that travellers are unaware that local laws apply to them and many British nationals think of their British passport as a 'get out of jail free' card.
"While consular staff will always try to assist British nationals who find themselves in difficulty abroad, we can't interfere in another country's legal processes.
"We want Brits to have a great time when they travel abroad so it is also a good idea to research the country they are visiting before they travel. Country-specific laws and customs can be found at www.gov.uk/foreign-travel-advice."