It was one of the most shaming, self-abasing apologies ever made in the House of Commons, indeed arguably in any western legislature. On Thursday, the attorney general read the prime minister’s statement saying sorry for Britain’s complicity in the abduction of a free man to live through six years of imprisonment and torture at the hands of a dictator, through which we hoped to gain information. His crime? He was the enemy of a murderous regime which realpolitik dictated we temporarily befriend.

Mrs May could scarcely have been more abject. “On behalf of her majesty’s government,” she wrote, “I apologise unreservedly… what happened to you is deeply troubling. It is clear that you were subjected to appalling treatment and that you suffered greatly.”

She acknowledged implicitly that Britain colluded in systematic torture. Britain was even party to extracting information via the torture process, although the statement stops short of linking the seeking of information with knowledge of torture. “We should have understood much sooner the unacceptable practices of some of our international partners. And we sincerely regret our failures,” May declared. “We shared information about you… we should have done more to reduce the risk that you would be mistreated. We accept this was a failing on our part.

“Later, during your detention in Libya, we sought information about and from you. We wrongly missed opportunities to alleviate your plight: this should not have happened.”

The dictator was Muammar Gaddafi; the man was the Libyan dissident Abdel Hakim Belhaj and the year was 2004. Tony Blair’s government slipped the whereabouts of Belhaj and his pregnant wife to the CIA, who abducted them and organised their dispatch to Libya. Belhaj’s wife, imprisoned in a nearby cell, could hear his screams; after giving birth she was released to live in Libya under surveillance, never leaving her house.

Tony Blair (left) shakes hands with the Libyan dictator Muammar Gaddafi in 2007. Photograph: Stefan Rousseau/PA

The allegation is that British agents were present at some of the torture sessions, hoping Belhaj could reveal possible links to al-Qaida.

Belhaj, hearing the news in Libya, was gracious. After six years, at last “justice has been done… A great society does not torture; does not help others to torture; and, when it makes mistakes, it accepts them and apologises. Britain has made a wrong right today, and set an example for other nations to follow.”

Here is hoping. It was because Belhaj refused a financial settlement, insisting on an apology while threatening a court case, that the scale of British involvement has been revealed – secured by the tenacity and skill of the human rights campaign Reprieve, and of Leigh Day, the law firm that represented him; and brilliant journalism from the likes of the Guardian’s Ian Cobain. Hats off to all of them – and to the British legal system that still confers justice, however slow, expensive and hard fought. That is not true of many countries.

Yet for all that, Belhaj – and the cause of the rule of law – got lucky. If the Gaddafi regime had not collapsed in 2011, the crucial cache of documents, exposing how MI6 had colluded with both the CIA and the then Libyan secret service, on which the case pivoted, would never have come to light. Thanks here to Human Rights Watch. We would have taken the government’s denials that it was in any way involved, and those in particular from the then foreign secretary, Jack Straw, at face value.

Huge issues now confront us. International law and the Geneva conventions ban the use of torture in any circumstance. The moral case is indisputable: it can never be right for one human being to abuse, inflict unbearable pain and abase another. There is also the practical case against torture. It actually undermines neurocognitive mechanisms by putting the brain under intolerable stress: the information that is provided is useless. It is for these moral, practical and international reasons that British law declares that evidence secured under torture is inadmissible – a view backed overwhelmingly by the British public in opinion surveys.

Yet Blair, Straw and MI6 contributed to the abduction of Belhaj, an act they should have known might lead to torture. There is an acutely embarrassing fax from Sir Mark Allen, the then head of counter-terrorism at MI6, to Gaddafi’s head of intelligence, welcoming the safe arrival of Belhaj. Blair told his inner circle that he was not going to make the same mistake as Harold Wilson in not siding with the US over Vietnam, and said publicly in 2005 that the rules of the game on human rights had changed.

Ken Macdonald, director of public prosecutions at the time, tells me that this attitude “fostered a shift in the moral landscape about our approach to national security. It came from a…wildly exaggerated view of the threat posed by al-Qaida to western nation states – degrading the agencies’ respect for traditional rights.”

Yet, by suspending our standards, and turning what should have been an aggressive international policing and security reaction to terror into a “war”, seismic mistakes have been made – colluding in torture; invading a sovereign country, Iraq, on the flimsiest of pretexts; and dissimulating, even lying, about what is going on to the public. Today’s widespread distrust of politicians and the carnage in Iraq and Syria, prompting the refugee crisis that has gifted European politics to the populist right, are rooted in those extraordinary years. We are paying a high price for setting aside our professed belief in the rule of law, human rights and behaving according to moral, even Christian, principles.

Straw’s mealy mouthed response was to declare: “I sought to act at all times in a manner which was fully consistent with my legal duties, and with national and international law.” He is not a bad man – but one profoundly compromised. A society, runs a Greek proverb, grows great when old men plant trees in whose shadow they know they will never sit. Straw should plant a tree. He should acknowledge the scale of what went on, that it was wrong and should never be repeated. Only thus is there any way back.