British diplomats will express with officials in Washington for a second time MPs' concerns about the treatment of a US soldier charged with leaking thousands of sensitive cables to WikiLeaks, the government has confirmed.

Foreign Office minister Henry Bellingham said staff at the British embassy in Washington would discuss Bradley Manning's detention with the US state department.

Bellingham made the promise after Labour MP Ann Clwyd raised the matter in parliament on Monday night.

Clwyd said Manning, who is charged with downloading 250,000 sensitive cables and passing them to WikiLeaks, had been stripped at night and held in solitary confinement for 23 hours a day.

His treatment at the Marine Corps brig in Quantico, Virginia made it more difficult for the US and Britain to campaign against human rights abuses in other countries, she said.

Clwyd, chair of the all-party parliamentary group on human rights, said that the UK's credibility was at risk in "places where human rights are not nearly so well observed."

She called on the government to offer practical support to the British relatives of Manning.

"I do not want us to get drawn into a discussion of the rights and wrongs of the WikiLeaks revelations. I would like us now to concentrate on the current conditions of detention for Bradley Manning," Clwyd said at the adjournment debate speech.

"Manning's case is important because of the message it sends out to the rest of the world about what kind of treatment the United States thinks is acceptable for people in detention. And, for us, it is important what we say – or what we don't say.

"That matters in places where human rights are not nearly so well observed. People will pay attention in China and in Russia – and in Libya, where we want to be on the side of those fighting for freedom from state repression.

"And most of all in Afghanistan: it matters to those UK and US service personnel fighting in Afghanistan what kind of image Britain and the US have in the world."

Clwyd drew on her experiences during the seven years she spent as special envoy to Iraq on human rights.

"It is my view that some of the greatest damage was caused to British and American efforts in Iraq when the stories of prisoner abuse emerged," she said. "It undermined our moral authority when we needed to explain that we were fighting for a better future for Iraq.

"The United States – and the UK, in the way we respond to actions of the US – needs to preserve that moral authority if we are to have a positive impact on the world and lead by example."

Praising the Guardian's coverage of Manning's treatment in the US, Clwyd said she would be willing to visit the solider if his family asked her to.

"I have read the several accounts of Bradley's treatment which have appeared in the press. Some very good accounts have been in the Guardian, and from David Leigh in particular," she said.

But the account to which she has paid most attention is Bradley's own, in which he complains of "improper treatment" and "unlawful pre-trial punishment".

Clwyd is the only politician to have directly questioned the foreign secretary, William Hague, over the government's position on Manning.

She has sponsored an early day motion calling for the government to raise the case with the US administration. The motion is supported by 37 MPs, including co-sponsors Peter Bottomley, Jeremy Corbyn, Mark Durkan and Paul Flynn.

Hague has previously said the government has not intervened because Manning's lawyer has said the soldier "does not hold a UK passport, nor does he consider himself a UK citizen".

"Our standing on this matter is limited," Hague said during a parliamentary debate last month. "[Manning] is not asking for our help, nor considering himself British."

But Clwyd has called Hague's response a red herring. She points out that Manning's mother, Susan, is Welsh and lives in Pembrokeshire, where Manning lived between the ages of 13 and 17, and she points to calls from Manning's lawyer, David Coombs, for his client's detention status to be changed.

Manning's treatment in the military prison in Quantico, Virginia, "ignores the repeated recommendations of the marine corps' own appointed psychiatrists", Coombs has said. His treatment "serves no purpose other than to humiliate and degrade Bradley Manning. I regard it as cruel and unnecessary."

Human Rights Watch has called on the US government to "explain the precise reasons behind extremely restrictive and possibly punitive and degrading treatment" that Manning alleges he has received.

Amnesty International has said "Manning is being subjected to cruel, inhuman and degrading treatment. This is particularly disturbing when one considers that he hasn't even been brought to trial, let alone convicted of a crime."

The UN special rapporteur on torture is understood to have raised his concerns with the US administration and is waiting for a response.

Clwyd quoted a recent column written for the Guardian by PJ Crowley, who resigned as spokesman for the US state department after criticising Manning's treatment, in which he repeated his conviction that it was "ridiculous, counterproductive, and stupid".