Three British men who have been held for seven months without trial on drug charges in Dubai were tortured by police with beatings and electric shocks, a human rights charity has claimed.

Grant Cameron and Karl Williams, both 25 and from London, and Suneet Jeerh, 25, from Essex, were arrested while on holiday on 10 July last year by police who claimed to have found a synthetic cannabis known as "spice" in their car.

The men signed documents in Arabic – a language none of them understands – after being threatened by having guns put to their heads and, in Williams's case, having electric shocks administered to the testicles, Marc Calcutt, a lawyer for the charity Reprieve, said.

They have denied charges of "consumption and possession with intent to distribute" and will appear at their first trial hearing on Thursday after spending seven months in custody.

In a draft witness statement provided to Calcutt, Williams said: "I remember that the police put a towel on my face so I could not see. They kept telling me I was going to die. I was so scared.

"Once I had been knocked to the ground, the police picked me up and put me on the bed. They pulled down my trousers, spread my legs and started to electrocute my testicles. It was unbelievably painful. I was so scared.

"Then they took off the towel and I could see that there was a gun pointed at my head. All I could think was that the gun in my face could go off if the policeman slipped, and it would kill me. I started to believe that I was going to die in that room."

The torture took place in the desert, it was claimed, where the men were initially taken after their arrest, and subsequently in a hotel room.

Calcutt said: "The idea that young British tourists on holiday can find themselves arrested and tortured in this way is truly appalling.

"The Dubai authorities need to immediately drop the charges against the men and conduct an independent investigation into how these terrible events occurred," he said. "If they do not, I am sure this story will linger in people's memories – particularly when it comes to booking their holidays."

Charities say the Emirati police are notorious for violence towards prisoners, including foreigners. Suspects are frequently beaten, starved, raped and force-fed drugs, they allege. Even accusations of trivial offences can result in foreigners being kept in cramped cells with up to 20 inmates for weeks and pressurised into signing confessions written in Arabic.

In 2011, Lee Bradley Brown, a 39-year-old British tourist, was beaten to death by officers in a Dubai police station after being arrested for swearing.

Brown was on holiday at a £1,000-a-night hotel in the UAE when he was arrested and kept for six days in Bur Dubai police station where, it is alleged, guards refused to give him enough food and water and did not let him see a lawyer.

Rori Donaghy, campaign manager for the Emirates Centre for Human Rights, said he is receiving an increasing number of reports about foreigners being tortured in police cells. "The United Arab Emirates know that their international trade is too valuable for there to be any global outcry over their human rights transgressions," he said. "[David] Cameron went there only last year to persuade them to buy Eurofighter Typhoon fighter jets in a deal apparently worth over £3bn."

The UAE embassy in London said it could not comment on the case; attempts to speak to police in Dubai were not successful.