Some countries impose the death penalty for drug offences
British travellers have been warned not to get involved with drugs abroad, as figures show a large number of those detained are accused of drugs offences.
Foreign Office figures for 30 September 2009 showed 1,057 of 2,582 Britons then jailed abroad were on drugs charges.
The majority were held in Spain, with 207, followed by the US, with 141, and Thailand and France, both holding 79.
Europe minister Chris Bryant said those arrested on drugs offences faced long jail terms in harsh conditions.
"The message is clear - we can't get people out of jail in other countries, so if you don't want to waste your life away in a tough foreign jail, be sensible and keep clean," he said.
Overall, the US detained the highest number of British nationals, with 669 in jail, followed by Spain, with 357, and Australia, with 271. Of the 2,582 detained in total, 256 were women and 2,326 men.
British nationals are being urged to respect the laws of the country they are visiting and be aware of the penalties they face if they break the law.
The death sentence can be handed down against people convicted of drug smuggling in China, Malaysia, Singapore and Thailand.
Other countries, including Cyprus, India, Venezuela, Ireland and the United Arab Emirates, can impose long sentences, even for small quantities of drugs.
The Foreign Office also warned that many people have an unrealistic expectation of what can be done for them if they are arrested abroad.
Father-in-law of man jailed abroad
Mark spent two years on remand in France before his trial in July 2008.
He was sentenced to four years in prison for drugs offences but released in December 2008 for good behaviour
The first prison Mark was in was very new and the guards were friendly and helpful. He got on well there.
However, three months before his trial he was moved to a very different prison near Marseilles. It had a completely different atmosphere.
It was filthy, very old and the guards were difficult. On his arrival Mark wrote to us asking us not to bring his little daughter out to visit anymore.
Being in prison abroad was extremely difficult and it put a lot of pressure on the family. However, we were very lucky that we could visit Mark relatively easily in France.
A survey suggested one in five Britons think diplomatic staff can get them out of prison, while others believe they can be automatically transferred to a domestic jail.
Just over half of consular time is spent supporting British nationals arrested and detained overseas.
Mr Bryant said: "Britons should know the risks of taking drugs abroad - and they should also be extremely wary of being sucked into the drugs trade.
"New figures show that over 1,000 Britons are in prisons overseas for drugs. People often don't realise that they will be sentenced in that country, that sentencing can take many months, prison conditions can be very difficult to handle and that sentences can be very long."
Pauline Crowe, chief executive of the UK-based charity Prisoners Abroad, said: "Conditions can be very poor in overseas prisons - particularly in South America, where a lot of people are arrested on drugs charges. Prisoners are often faced with severe overcrowding, poor quality food, water and medical care, and sometimes with violence.
"People may commit drugs offences for a wide variety of reasons, but we would encourage people to think of the consequences they will face if caught - the loss of liberty, the impact on their health, and the affect it will have on their family."