Parliament is seeking to clarify when compensation should be paid upon reversal of a conviction.

A miscarriage of justice – what is it and when should its victims receive compensation? Those deceptively simple questions were debated in the House of Lords last month. Not for the first time, the government did not get the answers it wanted.

Our starting point is the International Covenant on Civil and Political Rights, which the UK ratified in 1976. That requires states to compensate those who have suffered a miscarriage of justice. But in giving effect to the provision, section 133 of the Criminal Justice Act 1988 does not define its central concept – merely providing that compensation is to be paid when a conviction has been reversed ‘on the ground that a new or newly discovered fact shows beyond reasonable doubt that there has been a miscarriage of justice’.

What is clear from this wording is that you will not get compensation merely for being found not guilty at the end of a trial – even if you had been in custody since your arrest. And being found not guilty – or acquitted on appeal – certainly does not prove you did not commit the crime of which you were accused. That is why the courts had so much difficulty with the case of Nicholas Mullen, who was cleared on appeal of terrorist offences and then sought compensation. In 2004, Mullen finally lost his claim.

‘This was not because there was any doubt that he had committed the offences of which he was convicted,’ Lord Phillips said later in the Supreme Court. ‘His conviction was quashed because he had been seized and brought to this country from Zimbabwe in circumstances that had involved a flagrant abuse of power.’

So that was not a miscarriage of justice, the law lords decided. To find out what is, we must look at the case of Andrew Adams. Ruling on his appeal in 2011, a majority of the Supreme Court decided that section 133 certainly covered cases where a new, or newly discovered, fact showed the applicant to be ‘clearly innocent’. But it also included cases where the new fact so undermined the evidence that no conviction could possibly be based on it.

What was not covered were cases where something had gone wrong with the investigation or the trial; or where fresh evidence merely reduced the strength of the case. So although Adams himself had been cleared of murder on appeal, there had not been a miscarriage of justice in his case and he was not entitled to compensation.

In a later case called Ali, the divisional court reformulated the second test in Adams. The judges said there was a right to compensation for claimants who have ‘established, beyond reasonable doubt, that no reasonable jury (or magistrates) properly directed as to the law, could convict on the evidence’.

Two things follow. First, not everyone who is cleared on appeal will receive compensation. Second, not all those who receive compensation will be ‘clearly innocent’.

You can see why the government was reluctant to compensate people who might have been guilty. In its Anti-social Behaviour, Crime and Policing Bill, it proposed an amendment to section 133 that would have confined miscarriages of justice to cases where the new fact ‘shows beyond reasonable doubt that the person was innocent of the offence’.

Innocent? We do not require defendants to prove their innocence to the courts or to ministers. Common law, buttressed by article 6 of the human rights convention, presumes innocence unless and until a defendant is proved guilty.

Acknowledging that the government was more concerned about the cost of fighting compensation challenges in court than the cost of paying valid claims, parliament’s joint committee on human rights came up with a redrafted clause that would give effect, in clear terms, to the Supreme Court’s decision in Adams. The amendment would allow compensation ‘only if the new or newly discovered fact shows conclusively that the evidence against the person at trial is so undermined that no conviction could possibly be based on it’.

That amendment was moved by the independent peer Lord Pannick QC on 22 January, even though he is not a member of the human rights committee. It was supported by the two most senior retired members of the Supreme Court, Lord Phillips and Lord Hope, and by Baroness Kennedy QC (who cited An Officer and a Spy, the Robert Harris novel based on the Dreyfus affair). More to the point, Pannick’s amendment secured 245 votes, 23 more than the coalition whips could muster.

Speaking to me after the debate, Pannick said he hoped the government would not seek to overturn its defeat in the Commons. I share his view. What we have now is a clear, workable definition that will not attract human rights challenges or require ministers to assess a person’s ‘innocence’. If ministers cannot trust parliament and its committees, it should at least trust the judges.