Foreign secretary William Hague is considering making a dramatic public plea for the return of the last former British resident held inside Guantánamo Bay, as fresh reports indicate that the treatment of prisoners within the camp continues to deteriorate.

In the wake of President Barack Obama's renewed promise on Tuesday to close the prison, the foreign secretary has told MPs that he will escalate efforts to bring Shaker Aamer home to his family in south London.

Hague has yet to make a statement since Obama's pledge, but Tory MP Jane Ellison revealed that, during a meeting with MPs campaigning for Aamer's release, he had raised the option of upping the ante through a public plea.

Ellison said: "The foreign secretary was following Obama's comments with great interest and was considering making fresh approaches in light of those, possibly something public along the lines that Britain would like to be helpful in any way we can, the obvious way concerning you know who [Aamer]."

Concern is rising about the health of Aamer, who has spent more than 80 days on hunger strike. The US authorities admit that about 100 of the 166 detainees within the prison are currently on hunger strike, although lawyers estimate the true total is closer to 140, with a growing number, currently 21, being force-fed.

Aamer's lawyer, Clive Stafford Smith, said he was growing increasingly worried about the seriousness of his physical condition. Last week Stafford Smith twice attempted without success to contact Aamer via US authorities, prompting fears that Aamer, who is significantly beyond the point at which a hunger strike can cause "irreversible cognitive impairment", may be seriously ill.

As the pressure intensifies on Obama to act quickly on his promise to close the Guantánamo Bay camp, a pledge originally made during the 2008 presidential race, it has also emerged that the US president was recently rebuked by the head of the International Committee of the Red Cross (ICRC) over conditions inside the prison.

Peter Maurer, ICRC president, warned Obama in a meeting in Washington three weeks ago that the situation inside the facility was "not good". His concerns were echoed in subsequent briefings with defence secretary Chuck Hagel, senior CIA officials and the US deputy secretary of state, William Burns.

Sources added that levels of trust among detainees towards even Red Cross officials had plummeted to the extent that an unspecified number were refusing to be seen by the organisation's medical specialists. It has condemned the policy of force-feeding.

UN experts last week also strongly criticised the force-feeding of hunger-striking inmates, saying it was against international medical standards.

Pressure on Obama to realise his promise to shut the camp also surfaced in a open letter, published by the Observer, signed by 25 former Guantánamo Bay detainees. Together they call on the US medical profession to cease to assist with force-feeding.

Aamer, who has been held for more than 11 years, was cleared in June 2007. US documents dated November 2009 told him that the "United States government intends to transfer you as soon as appropriate arrangements can be made".

During the meeting with MPs last Wednesday, Hague also urged those present to use any contacts they had with Congress, which has blocked efforts to shut the prison, to help ensure Guantánamo Bay is closed as soon as possible. Ellison said she had already made contact with Democratic senator Dianne Feinstein. Hague and defence secretary Philip Hammond were, she added, still waiting for a formal response from the US to an approach made several months ago requesting the release of Aamer.

"All our efforts are now directed towards keeping the momentum going and reminding the president that the UK can help solve a bit of the problem for him," she said.

A Foreign Office source said it would be raising the issue of Aamer's release with the US at the highest levels at the earliest possible opportunity.