Prime Minister David Cameron is concerned about allegations that three Britons charged with drug offences in Dubai have been tortured, his office said, two days before the president of the United Arab Emirates visits Britain.
Grant Cameron, Karl Williams and Suneet Jeerh have been held in the UAE since taking a holiday there in July 2012.
Police said they had found a form of synthetic cannabis in their hired car.
The three were sentenced to four years in jail on Monday by the Dubai Criminal Court.
All three pleaded not guilty to charges of possession of illegal drugs and said police had subjected them to beatings and threatened them with guns, allegations the police deny.
In a letter to Reprieve, a London-based legal charity which campaigns for prisoner rights, Cameron said Britain had repeatedly raised concerns about the torture allegations with the UAE, saying the authorities' failure to organise a full medical examination of the men was worrisome.
"We continue to press for evidence of a full, impartial and independent investigation," Cameron wrote.
Authorities in the United Arab Emirates have dismissed the allegations, saying an internal investigation found them to be "baseless".
At a hearing in March, police officer Osman Ali Abdulla, who took part in the arrest, denied that any of the men were abused or beaten and said they were treated well.
Cameron's intervention on the eve of a state visit to Britain by UAE President Sheikh Khalifa bin Zayed al-Nahayan was welcomed by Reprieve, which has been supporting the three men, but is likely to irritate the Gulf state.
Britain is hoping the visit will boost trade between the two countries, which has in the past focused on defence contracts.
The UAE leader will arrive on Tuesday and be hosted by Queen Elizabeth at Windsor Castle, hold a meeting with Cameron, and have tea with the Prince of Wales.
He will be accompanied by a high-level government delegation during the two-day visit.
Reprieve said the case raised troubling questions.
"The prime minister's concern is welcome, but it is hard to see how a state visit is appropriate for the president of a country which has tortured our own citizens," Kate Higham, one of its investigators, said in a statement.
"At the very least, the mistreatment of these three men must be a central issue for discussion during the visit."
The remarks came as the mother of one of the men, Grant Cameron, described their "terrifying ordeal" following their arrest. Tracy Cameron told the BBC they were beaten and given electric shocks.
She said one of her son's friends, Karl Williams, "was laid out on the bed, his trousers were stripped down and electric shocks were administered to his testicles while he was blindfolded".
She said: "I believe all boys had guns held to their head -- they were told they were going to die."
There is zero tolerance for drug-related offences in the United Arab Emirates, a regional business hub and tourist destination where millions of expatriates live and work.
There are severe penalties for drug trafficking and possession.
Last year, a British citizen and a Syrian were sentenced to death by a UAE court after they were convicted of selling drugs to an undercover policeman.
The sentences were later commuted to four years in prison.