The number of computers and documents lost from secure government sites has almost trebled in the last year.
Data breaches leapt from 117 to 463 and missing electronic devices or paper documents rose to 62 from 22 the year before, an official Ministry of Defence report revealed.
It pointed out new rules mean more incidents are being reported – but the rise sparked alarm on Whitehall.
Experts fear sensitive information could find its way into the wrong hands as Russian online interference grows and China’s influence increases.
One Whitehall insider said: “At a time like this, as tensions run so high, any breach in security is dangerous.
“There’s always the possibility of things being stolen to order by foreign powers.
“But there’s also the risk of the wrong people coming across sensitive information by accident.
“What happens to it then? It could easily be sold to a state that is hostile to the UK, like Russia. We’ve already seen them operate here.
“And it’s not just foreign governments that would be interested in it. There are terrorist organisations who would be in the market as well.”
The figures were compiled while Gavin Williamson – now Education Secretary – was in charge at Defence.
He was sacked over suspicions he leaked details of National Security Council discussions about Chinese tech firm Huawei – which was bidding for UK communications contracts.
But MoD sources say tighter security will be a priority of new Defence Minister Ben Wallace.
Nia Griffith MP, Labour ’s Shadow Defence Secretary, said: “It’s very concerning to see sensitive documents or equipment go missing from secure locations, particularly as the UK faces a growing range of threats.
“The new Secretary of State must ensure his department does everything it can to trace these devices and prevent future security breaches.”
Andy Harcup, of data security firm Absolute Software, said: “Rising thefts of mobiles and laptops pose a serious security risk. Each device contains a goldmine of confidential data which could be exploited by hackers, foreign states or even a rogue employee.
“It’s vital all government organisations ensure devices are properly protected with end-point security, so they can track, secure and freeze them if they fall into the wrong hands.”