Poppy seeds in food, common over-the-counter medications and traces of banned substances are enough to warrant four-year prison sentences in the United Arab Emirates, travellers were warned today.

Visitors to Dubai and Abu Dhabi are now being advised to "take extreme caution" and "avoid arrest for 'possession' of a controlled substance".

The advice, issued by the legal charity Fair Trials International, follows a recent spate of arrests and imprisonment.

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The charity, which assists those facing trial abroad, has urged travellers to ensure they are completely free of any substances prohibited by the country they are visiting.

Earlier this week, a 43-year-old from Middlesex was imprisoned for four years after 0.003g cannabis was found in the tread of his shoe by customs officials in Dubai.

Keith Andrew Brown was stopped in transit from Ethiopia to London last September.

The amount of the drug found on his shoe would not be visible to the naked eye and weighs less than a single grain of sugar.

Fair Trials International Chief Executive Catherine Wolthuizen said: "We have seen a steep increase in such cases over the last 18 months.

"Customs authorities are using highly sensitive new equipment to conduct extremely thorough searches on travellers and if they find any amount - no matter how minute - it will be enough to attract a mandatory four-year prison sentence."

And the list of banned substances in the UAE includes many products which are available over-the-counter and off-the-shelf in the UK.

These include medications such as codeine, a common ingredient in pain relief and cold-and-flu medication, and the common baking ingredient, poppy seeds.

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Ms Wolthuizen added: "What many travellers may not realise is that they can be deemed to be in possession of such banned substances if they can be detected in their urine or bloodstream, or even in tiny, trace amounts on their person.

"We even have reports of the imprisonment of a Swiss man for 'possession' of three poppy seeds on his clothing after he ate a bread roll at Heathrow."

The UAE is becoming one of the world's most popular tourist and transit destinations - partly due to extensive marketing of its beaches and shops.

Charities now say travellers must be alerted to the risks they face if they are not completely clean of any banned substance or do not have a prescription for their medication.

Only last month a German citizen was detained for an alleged drugs offence when entering Dubai.

Cat Le-Huy, 31, was found carrying melatonin pills to help with jetlag and sleeping problems.

More than 2,000 people have now signed a petition asking for the British-based technical expert to be released.

Other recent cases include that of a 33-year-old Londoner who was arrested at Dubai airport last May after travelling to the UAE on business.

Customs officials found 2g of cannabis in his pocket which he had forgotten about.

He was sentenced to four years' imprisonment, but granted clemency and released in October 2007.

Customs officers in Dubai also alleged they found 0.03g of cannabis on Robert Dalton, 25, from Kent when he travelled to Dubai last year.

He was stopped and searched at the airport and arrested.

He is currently on trial and if convicted, will mostly likely receive a four-year prison sentence.

Tracy Wilkinson, 45 of West Sussex, was arrested at Dubai airport in 2005 for possession of codeine which she had been using to ease chronic back pain.

She was held in custody for 8 weeks before officials accepted proof from her doctor of its use for prescribed medical purposes only.

19 French nationals have been arrested in the past 18 months for possession of banned substances.

Fair Trials International are publishing a comprehensive list of banned pharmaceuticals on their website. This can be found at www.fairtrials.net .