NHS leaders have admitted that the biggest ever loss of patient documents is worse than initially thought, as another 162,000 undelivered documents have been discovered.

The scandal was first revealed back in February, when the UK's national health service was forced to admit that 709,000 items of correspondence – which includes details of patients' test results, change-of-address forms and other personal information – had gone undelivered.

The error by NHS Shared Business Services (SBS) – a joint venture between Steria and the NHS – meant that between 2011 and 2016, these documents were left gathering dust in a warehouse.

A team was tasked with investigating the incident, which included assessing whether the information had adversely affected patients' health, and it was thought that the situation was under control.

However, NHS England chief executive Simon Stevens on Monday told the Public Accounts Committee that some more undelivered records had turned up in the course of the investigation.

He said that, as part of the work, the team had looked at whether clinicians had stuck to processes introduced in 2015 that intended to improve the transfer of NHS documents – and discovered that there were about 5 per cent of cases "where that hasn't been happening".

Pressed on what this was in real numbers, Stevens said it meant there were about 150,000 more records that needed to be "repatriated" to the relevant GP practices.

On top of this, the team dealing with the incident investigated local offices across the country and found a further 12,000 SBS items languishing undelivered.

Karen Williams, the former director of transformation and corporate operations at NHS England (she now works at HMRC), said that this was because these boxes "had been assumed to be records for filing and therefore hadn't been processed".

Committee members were clearly exasperated by the latest admission, with chairman Meg Hillier saying that they had expected to "be beginning to wrap this up".

"We're very disappointed to still be discovering more problems," she added.

Geoffrey Clifton-Brown, meanwhile, expressed dismay that the execs had "started this hearing very confidently" when discussing progress on the initial tranche.

"Then you tell us this bombshell... what's the situation today for dealing with the backlog?"

In response, Stevens said that the team was applying the same triaging processes to the new records, which involved first making sure the relevant GPs received the records, and then having them vetted for clinically important information.

He said the NHS expected to have all the records back with GPs by the end of December for initial assessment, and that the end of March was "feasible" for finishing the whole project.

Of course, this extra work is going to cost. The government stumped up £2.5m to deal with the initial portion of documents, which is being used partly to fund GP practices that have to search through the medical records.

When pushed on the extra resources needed to deal with this final stage, Stevens said that he couldn't give a further number on it, but "would say in the zone of a million, rather than £2.5m".

Stevens also detailed progress on the original 709,000 items, saying that 5,562 cases had been sent for a full clinical review, and of these 4,565 had been completed.

Some 3,624 have been clearly shown not to have caused harm, with the remaining 941 awaiting a final clinical review. ®