The British Medical Association has written to Barack Obama urging him to immediately suspend the role of doctors and nurses in force-feeding prisoners held at Guantánamo Bay and to launch an inquiry into how the "unjustifiable" practice has been allowed to develop.

The association has also approached pharmaceutical companies that are reportedly supplying the US military with nutritional products used in the force-feeding, requesting that they disassociate themselves from issues surrounding the continuing hunger strike inside the detention facility.

The correspondence – sent to companies including Nestlé, whose product Boost Plus has been reportedly given to inmates being forcibly fed – says that "force-feeding of mentally competent adult hunger strikers is a gross violation of internationally accepted standards of medical ethics" and is "never ethically acceptable".

The BMA has also outlined its deep concerns to the US secretary of defence, Chuck Hagel, as international pressure continues to mount over the administration's handling of the hunger strike, which is understood to involve more than 100 of the 166 inmates.

Already ANI Pharmaceuticals, the US-based manufacturer of the drug Reglan, which is reportedly used on Guantánamo's hunger-strikers without their consent, has admitted that it is "deeply concerned" by the issue.

Responding to a letter sent by human rights charity Reprieve, which raised concerns over reports that Reglan is being used to treat nausea in detainees during force-feeding, Arthur Przybyl, chief executive of ANI, admitted that he was as "deeply concerned as you are by the complexity of the issues raised". He added: "Obviously it is our hope that all of our products are used in a medically acceptable manner."

Concerns over Reglan include claims that prolonged use of the drug can cause serious side-effects, including tardive dyskinesia, a neurological disorder which causes involuntary movements.

Reprieve has called on ANI to control the distribution of the drug and prevent it from being used in further force-feeding inside Guantánamo Bay.

Catherine Gilfedder of Reprieve said: "Reglan has been designated by the FDA as a harmful drug which must only be administered where patients fully understand and accept the risks. The policy of using this drug in force-feeding at Guantánamo adds a further level of horror to what is already a painful and degrading process."

Meanwhile, concern continues to mount that Shaker Aamer, the last British resident inside Guantánamo, might be on the brink of being transferred to Saudi Arabia. Aamer, whose family live in south London, has already been threatened by both the American and Saudi governments that he will be forcibly sent to the kingdom. The most recent incident was on 8 February, during which Aamer reports that Saudi officials told him: "When you are brought back to Saudi Arabia, you'll be interrogated, be tried, and you'll be jailed. Then you'll be put in the rehabilitation programme."

Signs are emerging that the Washington administration is again agitating over sending Aamer, a Saudi Arabian citizen, to the country instead of London, his preferred choice. An unidentified state department official was recently quoted as saying that it would be "unfortunate if a detainee didn't consent to a transfer to his home country, but that in and of itself wouldn't prevent such a transfer from happening".

In addition Aamer's family had been approached by a mysterious Saudi attorney, raising fears that attempts are being made for the family's lawyers to facilitate such a transfer. Clive Stafford-Smith, Shaker's lawyer and Reprieve's director, said: "Of course, the US has been a travel agent – the travel agent of shame, rendering Shaker and others all over the world against their will, to and from and via at least 54 countries that were complicit in torture and abuse. It is absolutely vital that David Cameron urgently acts to get Shaker back to his family in the UK before he is shipped off to Saudi to silence him for ever."