Foreign secretary William Hague has been accused of ignoring the plight of Bradley Manning, the British-raised US soldier accused of leaking secrets to whistle-blowing site WikiLeaks, by a former UK Special Envoy on Human Rights to Iraq.
Labour MP Ann Clwyd also accuses the Foreign and Commonwealth Office (FCO) of "continued stonewalling" of Manning's mother, Susan, whose son has been held US military authorities since he was arrested last May, according to a report in the UK's Guardian newspaper.
Susan Manning now lives in south Wales, where her son Bradley grew up after coming to Britain with his mother after his parents' marriage broke up.
She wrote to Hague three weeks ago, requesting that British consular officials visit him in military prison to check on his physical and mental health. Continued fears have been expressed over Manning's welfare after the 23-year-old soldier was placed on suicide watch.
So far she has received no reply from the FCO - leading Ann Clwyd to accuse the department of "playing some kind of avoidance game".
Critics of Manning's detention allege that his treatment constitutes torture, with reports indicating he has been forced to strip naked, woken repeatedly during the night and denied basic necessities such as bedsheets and pillows.
In spite of this, the British government was last week forced to admit that it had not raised Manning's case with the UN's special rapporteur on torture, Juan Méndez, after Clwyd tackled parliamentary under-secretary of state at the FCO, Alistair Burt.
Burt also refused to answer questions from Clwyd about Susan Manning's request for consular assistance.
"Susan Manning also asked for any help that could be given, in Washington and elsewhere, to the family if they so request it," Clywd said. "At the very least, Mrs Manning, who is very concerned by the situation of her son, should have had the courtesy of a reply."
Burt said the FCO is "limited in both what we can say and what we can do in this case" because Manning is alleged to have said he does not consider himself a UK citizen.
Clwyd called the response "disingenuous", pointing out that the FCO has already confirmed that although Manning does not hold a UK passport, he is British by descent.
"Their refusal to respond to Susan Manning or support Bradley Manning can't be [because of] a genuine confusion over his nationality, the responsibility the British government have for him or the conditions in which he is being held," added Cwyd.
Manning, who awaits trial on 36 charges for allegedly handing over US secrets, was recently transferred to a facility in Fort Leavenworth, Kansas, where conditions are said to be more relaxed.
The move follows widespread criticism of conditions at the military brig in Quantico, Virginia - including comments by former State Department spokesman PJ Crowley, who was forced to resign after calling Manning's treatment "ridiculous and counter-productive".