Hardly a day goes by, says Phil Shiner, without him or someone who works for him receiving abuse or worse.

“They will phone my office, sometimes daily. They’ll shout ‘c*nt’ to whoever answers the phone. I’ve had to train my team to stay calm and put the phone down.”

Has he had death threats? “Plenty, in letters, in packages in the post, in emails. People ask me, ‘Isn’t that what Pat Finucane did, he didn’t take the threats seriously either?’”

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At the mention of the Belfast solicitor shot dead in front of his family, there is a pause. Finucane acted against the British Army in human-rights cases on behalf of Irish nationalists, and was murdered by loyalists, possibly working in collusion with elements of the security forces.

Finucane had three children; Shiner has five. So why doesn’t Shiner stop? His answer is instant. “Principles. There’s something in me that says, ‘I’m not going to be bullied. They’ve picked on the wrong person to bully.’”

He’s a Catholic, a committed socialist, who believes passionately in social justice. Those are his guides.

Shiner, 57, is the Birmingham-based lawyer who is the scourge of the British Army, bringing case after case alleging brutality against Iraqi and Afghan prisoners.

Along the way, Shiner and his Public Interest Lawyers firm have earned the hatred of former and serving soldiers and their flag-bearers, including sections of the media. Richard Littlejohn in the Daily Mail accused Shiner of milking the legal-aid system, earning millions from the public purse to fund his cause: “Shiner is always on the lookout for a jihadist with a grievance which can be used to discredit the Army and win some hard cash.”

Last week, Shiner was on the receiving end again as the International Criminal Court (ICC) in The Hague announced it would launch a preliminary examination of claims that British troops committed war crimes after the invasion of Iraq. The court will study an estimated 60 alleged cases of unlawful killing and 170 of mistreatment of Iraqis in British military custody between 2003 and 2008. The ICC was responding to a complaint made by Shiner and the Berlin-based NGO the European Centre for Constitutional and Human Rights.

What was his reaction to the news? “That it’s a real breakthrough. Back in 2006, I pursued a much smaller number of torture and killing cases. Then the ICC said they felt sure war crimes had been committed, but they did not have enough of them. This one will involve the examination of far more cases and the prosecutors will have access to thousands of Ministry of Defence [MoD] documents. It’s a real breakthrough that vindicates 10 years’ hard work.”

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This is all said in a flat Brummie accent. It’s hard to imagine that in his spare time he is a comic. “Richard Littlejohn said I wasn’t known for my sense of humour. When I did my stage show, I got 37 laughs in five minutes. I’ll stand by that.”

The UK has its own investigatory body, the Iraq Historic Allegations Team (IHAT), set up by the MoD. Shiner has little faith in the IHAT. “I’ve no confidence in them. They’re interviewing, but slowly; they’ve had some of the cases 10 years, and there are hundreds of them.”

He’s optimistic of the ICC now getting to grips with the claims, because “the ICC is not woven into our political establishment”. It’s been a rocky road, he admits, but insists, “I’ve never stopped believing in the rule of law.” He hesitates. “If I did, I might as well take up market gardening.”

He has no doubt where it could lead. “All levels, right up to Geoffrey Hoon [the Defence Secretary during the invasion], and those who followed him as Defence Secretary while we were in Iraq, but also the chiefs of intelligence and general staff. It should take in those at the very top, as well as the interrogators below.”

He draws breath. “I believe there are at least another 11 Baha Mousas, 11 other deaths in British custody.”

It was Shiner who doggedly pursued the case of Baha Mousa, the Iraqi hotel receptionist tortured to death by British troops in 2003. One soldier, Corporal Donald Payne, admitted to inhumane treatment of detainees and was imprisoned for one year. He has the distinction of being the only British soldier in history jailed for a war crime.

Six of his colleagues were acquitted and, while the judge was certain Mousa had been subject to sustained assaults and maltreatment over 36 hours, “a more or less obvious closing of ranks” meant other charges had to be dropped.

Shiner is determined to go further. “I’m not going to rest easy until I find out the answer to the question of exactly how many died in British custody.”

It would be far better, he says, if the MoD ceased resisting at every turn. “The MoD needs to stop, accept it and grow up. They’ve been rumbled, they’ve got it wrong. This wasn’t one-off behaviour, it was systemic and they know it.”

His proof is the manuals from the Army’s intelligence and training centre at Chicksands in Bedfordshire. “They show that the training was blatantly unlawful. It’s like they reverse-engineered the ‘Muslim Handbook’. Sexual humiliation plays a big part. They must get the prisoner naked, so the interrogator pretends they must strip for a medical examination.”

He scoffs. “Honestly, what sort of nonsense is that?”

In one of the cases, the prisoner was made to take his clothes off in front of a lot of people, including women, and made to kneel on all fours. “One of the soldiers gets an erection and says ‘jiggy-jiggy’. He stands by the naked man and masturbates on his back.” In another, “a prisoner had hardcore porn played to him all night. He was asked if he’d masturbated.”

And there was “a woman interrogator who straddled a man, and put her genitals in his face. She said, ‘Do you want that? You can have it if you tell me what I want to know.’”

This, and more, with names, he hopes will come out now the ICC is involved. “It’s part of the training regime at Chicksands – get them naked and keep them naked. The MoD says it’s not systemic but it clearly is.”

His “ultimate goal”, Shiner says, is to achieve “reform of our armed-services personnel so it never happens again. We need to train soldiers and interrogators properly, train them in the basic elements of law.”

He can’t see why the Army can’t be like the police. “If you join the police and you patrol the streets you need to know what the law is. That doesn’t happen in the Army – they’re not taught it.”

He sticks with the police comparison. “It’s very simple: why do we need interrogators to do disrespectful things to people? We don’t. A skilled police questioner knows, when a child has gone missing, if the person in front of them knows where he or she is. They can tell. They know if they’ve got the right person. And they’ve done that without doing anything disrespectful.”

What about the charge that he is unpatriotic? He bristles. “What, that I don’t care about the soldiers? What nonsense. I acted for the Gurkhas from 2002 onwards. In one case I secured £40m in compensation for the Gurkhas captured by the Japanese. The Gurkhas were excluded by the Government from the compensation scheme. I got them compensation.”

He’s acting for one of the mothers of the six British military policemen murdered in Iraq in 2003. And he’s represented the Gulf War veterans affected by Gulf War syndrome. “The list of people I’ve acted for who are serving or ex-serving military personnel is a long one.”

And is he making millions at the taxpayers’ expense, as Littlejohn alleges? “I’ve put in hundreds of thousands of hours into the ICC complaint at my own expense and my team’s expense. We’ve spent a small fortune putting it together. But we’re doing it for free. Why? Because we might be a dying breed, because we’re lawyers of principle. The idea I’ve enriched myself doing this sort of work is nonsense.”

But he’s made his point; why not let them rest? Is he not damaging the nation by constantly bringing these cases? “Listen, the people who are damaging the nation are the senior politicians, civil servants and lawyers who let this happen. Those are the people who are damaging the nation.”

In 2003, he says, “We went into Iraq to win hearts and minds. How could we do that, if people were taken into custody and came out in body bags? We abused women and kids, and expected people not to talk to each other! Well, they did.”

It’s been referred to, he says, as a loss of a moral compass: “But moral compass doesn’t cost money. It’s something drilled into you by training and discipline. The troops should have been told by their commanding officers: ‘There’s a line in the sand and if you cross it you will be found out. You will be dishonourably discharged at best and your Army career will be over. You will have brought shame upon your regiment. You’ve a duty to report to me if anyone crosses that line in the sand.’ With the right ethos and training, the commanding officers could have made sure that ethical principles were applied.”

There is such a thing, I venture, as the “fog of war”. His answer is mocking. “People who say that don’t know what they’re talking about. Iraq wasn’t a war – the fighting lasted just five weeks. After that, we were an occupying power. Afghanistan was not a war. It was a United Nations operation to assist the government there to restore peace to the region.”

Our bad behaviour, he says, has done us harm: “Look at us. We’re supposed to be a democratic nation, a great democratic state that abides by the rule of law. Now countries can turn around and say, ‘No you don’t – you’re not better than us.’ The Syrian government can say, ‘Don’t you dare lecture us on what we would and should not be doing.’”

The blame, he believes, lies with our inability to confront our colonial past, to realise our empire has gone, and that when we had the empire, we did some terrible things. “Nobody has ever made the British public face what their armed forces did in the Second World War and the colonial wars since,” he says. “Germany was divided into four, made to eat humble pie, and national monuments were built commemorating their atrocities. What have we ever done? Nothing.”

He must go. He’s got a “highly confidential” meeting to attend, this time involving the rapes of women. Seemingly, his work never ends. And whose fault is that?

Phil Shiner: The essentials

Born: 1956, in Coventry

Education: Bishop Ullathorne Roman Catholic Comprehensive School; Birmingham University (law)

First major victory: defeats Edwina Currie as a member of Birmingham City Council over a policy she was promoting of housing tenants behind in their rent in a “punishment block” of an old barracks

Moves: from law centre in Birmingham to Barnardo’s in Bradford as a community-development worker, to Birkenhead working for the Labour MP Frank Field. Sets up environmental-law department for Tyndallwoods law firm in Birmingham

Finally: in 1999, launches Public Interest Lawyers

Family: twice married; three children from his first marriage, two from his second, to Rachel, a gastroenterologist

Relaxes: runs (once did a marathon in two hours, 30 minutes), cycles, goes on an annual religious retreat to Iona