MPs investigating the biggest ever loss of NHS medical correspondence have been told that 162,000 extra documents went missing, beyond the 702,000 pieces of NHS correspondence already known to have been undelivered.

Over half a million of the items of correspondence, the majority containing patient identifiable data, that piled up undelivered from late 2013 but it took over two years for the alarm to be raised or effective action begun. NHS England chief executive Simon Stevens told MPs that it will be March before a full review of correspondence is completed, nearly five years on.

The missing documents came to public attention in February and resulted from outsourcing firm Capita and NHS Shared Business Services (SBS) failing to deliver vital medical correspondence, and proving incapable of dealing with a massive backlog that it allowed to build up over several years.

A subsequent report by the National Audit Office (NAO), published in June, identified at least 1,700 cases of potential patient harm resulting from the medical correspondence scandal.

The NAO investigation found that managers at SBS had been aware of a clinical risk to patients since at least January 2014 but did not develop a plan to tackle the backlog. NHS England estimates that handling the incident will cost £6.6 million for administration alone.

News of the latest losses came at an oral evidence session of the Commons public accounts committee, which is investigating the disappearance and storage of the patient documents.

According to a 17 October report by the Guardian NHS officials told MPs said that in the course of their inquiries, they had identified a further 162,000 medical documents that had either not been delivered or processed by Capita and SBS.

NHS officials told MPs that the latest batch of lost documents could included time critical treatment plans, details of changes to what drugs patients should be taking, child protection information and the results of a range of diagnostic tests.

Since the backlog and missing files came to light NHS officials have been investigating cases in which patients may have been harmed as a result of the medical correspondence not being delivered, and some 5,562 cases have been through clinical review.

Of those, 4,565 have been completed – 3,624 of which have shown that there was no clinical harm to patients. The NHS senior officials told MPs there are 941 cases still to be cleared, and said that no one has yet been identified as coming to harm because of the loss.

Simon Stevens told MPs on the PAC: “We think there are probably about 150,000 items or so that require repatriation back to GPs and we aim to do that by the end of December.”

The NAO found that SBS first recognised in January 2014 that patients may have come to harm due to a rising backlog of undelivered correspondence, but did not alert the department or NHS England until March 2016, over two-years later.

Vince Cable, the Liberal Democrat leader, called on Secretary of State for Health Jeremy Hunt to urgently appear before MPs to update them on the situation.

“This colossal blunder now turns out to be even worse than previously thought. the safety of thousands of patients has been put at risk due to the incompetence of a single private company and lack of proper oversight. Jeremy Hunt must urgently come before parliament to explain what steps are being taken to ensure this does not happen again.”

NHS SBS is a jointly owned and run by the Department of Health and Sopra Steria, an IT consultancy company that describes itself as a European leader in digital transformation. The company also has a joint venture with the Cabinet Office.

The company’s website says of the two UK joint ventures, “Both demonstrate our experience in making shared services work, at scale, to deliver significant savings.”