Defence secretary Ben Wallace was accused of appearing to condone the use of mock executions after comments he once made that soldiers “might pretend to pour petrol” over prisoners taken on the battlefield resurfaced.

The former Scots Guard captain and future cabinet minster had declared that “battlefield short sharp interrogation where the prisoner is manhandled fairly roughly … is absolutely the norm” to a newspaper in 2003.

Trying to obtain information at the point of capture was critical, Wallace told the Scotsman in the immediate aftermath of the Iraq war. “It’s taught to soldiers that’s how it’s done. You might pretend to pour petrol over them, when it’s actually water.”

The remarks, made at a time when Wallace’s political career was in its early stages, were unearthed by human rights group Reprieve, which accused the defence secretary of appearing to condone “illegal, immoral” techniques.

Maya Foa, director of Reprieve, said: “It’s hard to see why Wallace would pretend he was pouring petrol on a detainee other than to make them think they were about to be burned alive, which quite obviously amounts to a mock execution.”

She added: “The behaviour Mr Wallace is endorsing is not simply ineffective, it is illegal, immoral, and undermines principles which generations of British service personnel risked their lives to protect.”

Mock executions were amongst the torture techniques used by the CIA during the post 9/11 “war on terror”. They were exposed and criticised by a landmark US Senate report in 2014 for being brutal and ineffective.

The current Ministry of Defence guidance on captured persons does not specifically mention mock executions in the list of practises that it rules out. But it makes clear that UK armed forces are required to “act humanely” when somebody has been seized.

The 2015 document adds: “There are no circumstances in which torture, cruel, inhuman or degrading treatment can ever be justified,” and that “cruel treatment and torture” is absolutely prohibited at all times.

Torture is also expressly prohibited in the 2010 consolidated guidance for UK armed forces and intelligence personnel. But in May, Wallace’s predecessor Penny Mordaunt ordered a review of the MoD torture guidance after it emerged that it had been discreetly rewritten.

The new version allowed ministers to override the rules where there was a risk that intelligence obtained from allies may have been obtained via torture - if “the potential benefits justify accepting the risk”.

Wallace served in the British army in the 1990s, after completing his training at the Sandhurst officers academy. He served in Northern Ireland – where he was mentioned in dispatches in 1992 – and in Germany, Cyprus and central America.

At the time of the interview in May 2003, Wallace had already embarked on his political career. He had just completed a four-year term as a member of the Scottish parliament.

He first became an MP in 2005 and was promoted by Boris Johnson into the cabinet as defence secretary in July. One of his first acts in the job was to say that veterans who served in Northern Ireland should not face prosecution unless new facts emerged.

In the original newspaper interview, Wallace sought to justify why he believed such practices were necessary. “I would defend the right of soldiers to interrogate at the point of capture, swiftly and sharply because that’s when you get the most information.

“That is not a war crime. Where there’s still a threat to our forces, you need to know information quickly. If that man had been given a cup of tea, would he really have told about his weapons under the floor?”

Opposition politicians criticised his remarks. Stewart McDonald, the SNP’s defence spokesperson, said: “The use of torture is never justified and any attempts to normalise – or indeed justify – these deeply troubling remarks, especially by someone who was installed by Boris Johnson, is deeply concerning.”

McDonald also called on the prime minister to distance himself from Wallace’s comments and said “given the department’s history of freelancing on torture policy” the defence secretary should not sanction any behaviour that breaches UK and international law.

The Ministry of Defence was contacted for comment.