The Atomic Weapons Establishment says it may hold nearly 5,000 personal blood tests of troops irradiated during nuclear weapons tests.
But the AWE said it could not be certain what was in the files, because checking them would be too expensive.
It comes after five years of repeated denials by the AWE, MoD, and ministers that they hold any such files, and follows a meeting in which Veterans Minister Johnny Mercer told campaigners that if they did not believe the claims, they could sue the government to get the truth.
Defence Secretary Grant Shapps has ignored requests to meet campaigners, and Prime Minister Rishi Sunak has told them he is too busy. If blood tests exist, they could prove whether troops were irradiated - leading to multi-million pound compensation payouts.
Disabled dad Steve Purse, whose father had blood tests taken during his service at a series of toxic radiation experiments in the Australian Outback, has been refused access to the records, even though it might help his diagnosis.
Teachers, civil servants and train drivers walk out in biggest strike in decadeHe said: “These files the AWE now admit it has could be the answer to my family’s questions about what happened to dad, if the radiation got inside him, and if it’s responsible for my problems. They've told me and my mum they have blood tests, but won't let us see them. "
He adde: "To refuse to hand them over on the basis that looking for them is too expensive is like saying we don’t matter. That we're too expensive. It’s exhausting to just be re-victimised, over and over again.”
A lawsuit has now been launched by human rights firm McCue Jury & Partners, and a £100,000 crowdfunder set up to bring it to court.
In 2018, the MoD told Parliament it was "unable to locate any information that suggests... staff took blood samples for radiological monitoring at the tests. Service personnel who were present may have had blood samples taken during their career, but these individual military medical records are not held centrally."
Last year the Mirror revealed a 1958 memo between atomic scientists discussing the “gross irregularity” found in the blood of Squadron Leader Terry Gledhill, who was ordered into the mushroom clouds on sampling missions. We uncovered Whitehall instructions for widespread blood testing, internal discussions about how to do it, and 150 hidden documents in AWE archives with titles including "blood counts", "blood examinations" and "blood tests". The government has refused requests from MPs and peers to publish them.
In Freedom of Information requests, we asked the AWE - an arm’s length agency of the MoD - to define exactly how many veterans' blood tests it held.
At first we were told there were “a small number”, then “one blood test”, and that there were no blood records in its archives. After a complaint to the Information Commissioner, the AWE admitted it holds 4,711 files “which may contain references to blood or urine testing”.
It said each file would need to be individually examined to determine its contents, and at an estimated 15 minutes to check each record, it would exceed the costs limit set under FoI laws.
Veterans who have requested their personal data from AWE are repeatedly told it has none, and their medical records are found to have large sections missing. Families, including widows and executors with a lawful right to access a deceased person's medical records, have been refused access to them.
After campaigners presented Mercer with a dossier evidence earlier this year, he told them that he’d been assured by the MoD there was no issue, and said that if they wanted to take it further they could sue.
Richard 'shuts up' GMB guest who says Hancock 'deserved' being called 'd***head'A MoD spokesman said: “It remains the case that no information is withheld from veterans and any medical records taken either before, during or after participation in the UK nuclear weapon tests are held in individual military medical records in the Government's archives, which can be accessed on request.”
He added that “several of the documents may contain blood test data, but this is never attributable to an individual” and insisted the AWE held no medical records.
The Mirror has multiple archive documents with blood and urine tests attributable to individuals, and confirmed we have them to the MoD. Last week, a defence minister was asked how veterans could request their blood tests, and he replied they could ask the AWE.
The number of files AWE says it would need to check corresponds with the number of veterans' radiation dose records the government has previously admitted retaining.