Hunger strikers at Guantánamo Bay are being threatened with body cavity searches before they can see their legal representatives, a leading human rights lawyer has claimed in a letter to British foreign secretary William Hague.

Clive Stafford Smith, the founder of legal group Reprieve, represents various detainees in Guantánamo, the controversial US military detention camp in Cuba used to house terrorism suspects.

They include Shaker Aamer, who is among more than 100 inmates currently on a hunger strike at the jail that has created global headlines and a growing political problem for President Obama by focusing on attention on how so many inmates have been detained without trial for so long.

In the letter to Hague, seen by the Guardian, Smith said US guards insist on a body search before any detainee can contact their lawyer, either via an in-person interview at the base or with a phone call. "The US military has started directly abusing prisoners who want to contact their lawyers to tell them what is happening. So anyone who wants to see a lawyer, or have a legal phone call, must have his fingers put up his anus and his genitals touched," Smith wrote.

Smith added that as a result of the threat, two of his clients and at least one other prisoner refused to go through with the search and thus missed a chance to talk to their lawyers. The claims also match reports by the Agence France-Press news agency, which has reported that prisoners must undergo the searches before and after talking with lawyers. "Any pretext given for these new rules is just that: a pretext. The prisoners do not need to be sexually assaulted in order to be taken to a telephone to talk to their lawyer," Smith wrote.

The claims were denied by the US military. "Full frisk searches are conducted in a professional manner to quickly locate and identify contraband hidden on the body. The searches are conducted with clothes on, similar to a pat-down search conducted by an airport security screener," said Lt Col Samuel House in a statement.

House added that detainees were searched when they moved between facilities and that any refusal would result in them not being escorted to any appointment. "If a detainee refuses to be searched as part of a routine move outside of camp, such as for a phone call, legal appointment or non-emergency medical appointment, the guard force will not take the detainee," he said.

Aamer, a British resident, has been held at Guantánamo for 11 years, despite never having been tried or charged. He has twice been cleared for release, but remains behind bars as the last Briton at the camp. The issue of his continued incarceration has seemingly become a sore point between the UK and US governments.

The British government maintains it is committed to getting Aamer out of Guantánamo. But that official stance has seemingly been undermined by the Pentagon, with officials reportedly briefing that Britain's commitment to the detainee's release is half-hearted. When the senior senator from New Mexico, Tom Udall, recently enquired into the status of Aamer after being lobbied by his supporters.

What he was told by an official at the Department of Defense's legislative affairs division differed substantially from the long-held British line. An email seen by the Guardian from Udall's chief of staff, Michael Collins, to advocacy group Code Pink's founder Medea Benjamin stated: "We were told that the UK is not exactly in a rush to get him."

That stance has been denied by British officials. However, in a separate letter to Hague, Smith said Pentagon officials had said that the revelation of the differing messages had caused consternation among British officials. He quoted Tara Jones, an assistant to senior Pentagon official William Lietzau, as saying the reports had caused a "huge blow-up with the Brits".