Human rights lawyer Martyn Day admitted his firm’s failure to disclose a key document earlier was ‘inexplicable’, the Solicitors Disciplinary Tribunal heard today. Day, senior partner of London firm Leigh Day, told the Al-Sweady Inquiry into allegations of abuses by British troops that he could not say why the firm had held onto the document – known as the OMS list – for nine years.

The tribunal heard that a translation of the one-page document written in Arabic was made after it came into the firm’s possession in 2004. The list linked detainees represented by Leigh Day in claims against the British Army with members of the insurgent Mahdi Army, and was accepted as being hugely damaging to the case.

Timothy Dutton QC, for the Solicitors Regulation Authority, said the document was of ‘obvious importance’ and that an experienced and highly skilled lawyer would have been expected to read it.

‘If he [Day] had been paying sufficient attention to his files he would have read the document,’ said Dutton. ‘The conclusion one draws is he was not paying sufficient attention to the files and never properly read the OMS detainees list.

‘The alternative explanation is that Mr Day did read the document but in a frame of mind where he did not appreciate its significance.’

Dutton stressed the SRA was not suggesting Day had been dishonest but that he was ‘pre-disposed’ towards wanting to accept his clients’ account and was not approaching the case with ‘openness and an inquisitive mind’.

Sapna Malik, a partner at Leigh Day, was in possession of the list during fact-finding trips to Syria and Turkey, but she denied she could have spotted its significance earlier than 2013.

The tribunal heard the firm recorded 1,400 hours in preparation time for its claims against the Ministry of Defence and Dutton said the failure to review documents was ‘very serious’.

He cited an internal email sent within Leigh Day which said the non-disclosure was ‘potentially the most damaging issue’ it faced and showed a ‘total failure to control’ how cases were being handled.

The tribunal hearing continues. All respondents deny wrongdoing.