Jack Straw has called for US-held al Qaeda suspects to be treated "humanely and in accordance with customary international law".

The foreign secretary moved on Sunday to soothe growing unease among Labour MPs about conditions facing the prisoners - some of whom claim British citizenship - as photographs showing shaven-headed, kneeling and bound suspects dominate the newspapers.

An announcement that the US plans to extend the controversial Guantanamo detention facility from 110 prisoners to 320 until a prison holding up to 2000 is constructed is set to intensify human rights concerns.

"The British government's position is that prisoners - regardless of their technical status - should be treated humanely and in accordance with customary international law," he said in an official statement. "We have always made that clear and the Americans have said they share this view."

But speaking to the BBC, the former foreign minister Tony Lloyd expressed his concern that "the treatment does seem to be way below the standards we would expect".

"Are those prisoners going to be held up to the standards of the Geneva Convention, because that standard was set during a period which has seen us take in world wars where people were guilty of the most horrendous crimes, but we insisted that the Geneva Convention was the minimum standard. to provide a floor below which civilised nations shouldn't fall," he said.

"And frankly, Britain is a civilised nation; we must insist ourselves that we abide by the Geneva Convention, and we have got to insist that our allies, and we were America's closest ally, stick by that minimum standard."

MPs on the all-party Commons human rights select committee are seeking a meeting with the US ambassador, William Farish, to raise their concerns over the treatment of prisoners.

Joint committee chairman, Ann Clwyd, believes it is "playing with words" to deny that the detainees were not prisoners of war.

Clwyd aims to meet Farish "to voice concern about the treatment of all the people taken prisoner in Afghanistan, not just the ones being held in Cuba, and we want an acceptance that they are prisoners of war".

"I was in Geneva this week and saw Mary Robinson, the UN's high commissioner for human rights, and she was quite clear that they are prisoners of war. If there is a dispute about that, that dispute should be settled by an independent tribunal, not by Donald Rumsfeld (US Defence Secretary) nor any single individual," she told the BBC.

"You can't play around with human rights and the rights of such prisoners are set out in the Geneva Conventions which both the US and ourselves are signed up to."

The backbencher warned ministers that there was "substantial concern" among her colleagues and insisted that the UK be firm with the US.

"We fought the war shoulder to shoulder, now it seems that we are being frozen out of the aftermath, she added. "It's time we became much firmer with the Americans and insisted on the human rights we are all signed up to."

Lib Dem foreign affairs spokesman Menzies Campbell said that the detainees must be told their rights if they are to be interrogated about involvement in terrorist activities - as would be normal in the UK.

"The sort of circumstances in which [an intelligence investigation] might happen in the United Kingdom would always be accompanied by the individual having some idea of exactly what his rights might be," he said.

"That's why it seems to me that before any intelligence officers begin what may be a perfectly legitimate investigation, then the people concerned ought to be given some indication of what their legal position is, and the extent to which they are obliged to answer any questions."

Speaking to Sky News, Campbell went on to caution Western leaders that the photographs of cowering and shackled prisoners were "undercutting the moral high ground that both Mr Bush and Mr Blair enjoyed as a result of their robust response immediately after the 11th of September."

"You only have to ask yourself the question what sort of effect will these pictures have in capitals like for example Cairo, or Amman in Jordan. They will have a very considerable influence on public opinion," he said.

But Conservative leader Iain Duncan Smith warned against getting "overly exercised" over the rights of suspected terrorists who might have been involved the September 11 attacks on the World Trade Centre.

"Many of these men are among the most dangerous people you will ever find anywhere in the world before we all get overly exercised about this let's just pause for a second and say 'let's get the balance right, let's find out what the circumstances are'," he told the BBC.

"These people are immensely dangerous people and we must make certain that they are held securely and not able to cause any death or mayhem again."