The government has been forced to admit to another possible data loss - at the agency responsible for paying subsidies to farmers whose IT system was such a disaster that the UK had to pay £70m in fines for failing to meet EU targets.

Environment, food and rural affairs secretary Hilary Benn was forced to rush out a Commons statement on the concealed loss at the Rural Payments Agency (RPA) the day before it was to be revealed in a report in Farmers Weekly.

Benn told MPs he had only learned the day before that 38 data backup tapes and one compact disk had been "unaccounted for".

He said 35 had since been found and he claimed that of the four remaining missing, a tape and the CD did not contain "personal protected data".

Of the final two which "potentially contained partial data in code", one was faulty and unable to be read and the other required specialist equipment and detailed technical knowledge.

Benn said: "I want to reassure farmers that there is no evidence that the tapes are in the public domain."

He said there had been "a forensic investigation" and officials had concluded there was "only a low risk" that any personal usable data had been lost and "a judgement was made in accordance with Cabinet Office guidelines that ministers need not be informed".

But Tory shadow environment secretary Nick Herbert challenged the decision not to inform the pubic. He said it "looks like a cover-up" and demanded Benn " accept responsibility for another foul-up by the RPA".

The row erupted following a second National Audit Office report on the RPA complaining, according to one MP, that it "had scant regard for public money" with a high cost base and demanded Benn "take responsibility for this shambolic organisation".

Benn admitted its history "is not a happy one" but claimed it is now getting payments to farmers more quickly and he did not want to do anything to undermine that.