The cases of 173 patients are being urgently examined to see if they have suffered harm after the loss of 500,000 pieces of NHS data, MPs have been told.

Family doctors have told the Department of Health (DH) that the cases “require further clinical review”, in what a senior Conservative MP called a “very serious incident”.

However, Health Secretary Jeremy Hunt has denied Labour claims of a “cover-up” and insisted no evidence has yet been found of patients put at risk.

Mr Hunt was dragged to the Commons chamber to explain himself after The Guardian revealed what is believed to be one of the biggest losses of sensitive clinical information in the NHS's 69-year history.

The 500,000 items of data sent between hospitals and GPs went undelivered over the five years from 2011 to 2016, including test results and treatment plans.

The private company meant to ensure delivery - NHS Shared Business Services (NHS SBS) - mistakenly stored the records in a warehouse, it emerged.

In the Commons, the Health Secretary denied any data had been “lost” - because it had been safely stored in the warehouse, enough though it had not been delivered.

And he insisted he had not gone public about the blunder, in March last year, on the advice of officials who feared GP surgeries would be swamped.

“Publicising the issue could have meant GP surgeries being inundated with inquiries from worried patients which would have prevented them from doing the most important work – namely investigating the named patients who were potentially at risk,” Mr Hunt told MPs.

However, as Mr Hunt was speaking, the Commons Public Accounts Committee (PAC) released a letter from the DH’s permanent secretary, sent last Friday.

It stated: “The NHS England Incident Team has now completed distribution of around 2,500 documents triaged as potential high risk of harm.

“173 responses have been received so far from GPs indicating cases that require further clinical review and arrangements are now being progressed for relevant clinicians to conduct these reviews between now and March 2017."

Mr Hunt made no mention of the 173 cases as he answered an urgent question, even while admitting it was “completely extraordinary” that the blunder had not been spotted for five years.

Labour accused Mr Hunt of a “catastrophic breach of patients’ data” that had put safety at risk, adding” “Patients deserve answers and they deserve an apology.”

Sarah Wollaston, the Tory chairwoman of the Commons Health Select Committee, warned of a “very serious incident”, but praised Mr Hunt for his response.

But the Health Secretary insisted: “No patient data was lost and all patient data was kept in secure settings

“As things stand, there is no evidence so far that no patient safety has been put at risk.

“There are 2,500 [documents] where we are doing a second check with GPs – a second legal opinion is being sought - and, in nearly 2,000 cases we do not think there is any evidence. We are going through the remaining ones.”

The data included the results of blood and urine tests, and of biopsies and screening tests for diseases including cancer.

It also included letters containing details of patients' visits to hospital, including to oncology clinics, and information about what they had been diagnosed with after visiting accident & emergency.