The armed forces are ill-prepared to defend the UK against a serious military attack, a senior commander warned the defence secretary.
In a memo before he retired in April as head of Joint Forces Command, Gen Sir Richard Barrons said key capabilities had been stripped out to save money.
He said Whitehall was “preserving the shop window” with items like aircraft carriers, the Financial Times reports.
Defence officials said Sir Richard had backed the last defence review.
The FT said it had obtained the 10-page private memo, which had been sent to Defence Secretary Sir Michael Fallon.
It followed the government’s decision to raise defence spending by nearly £5bn by 2020-21 and its pledge to meet Nato’s target to spend 2% of GDP on defence for the rest of the decade.
But Sir Richard warned that “capability that is foundational to all major armed forces has been withered by design”.
He said critical technical and logistical capabilities have been “iteratively stripped out”.
“Counter-terrorism is the limit of up-to-date plans and preparations to secure our airspace, waters and territory,” he said.
Referring to defence of the UK, Sir Richard continued: “Neither the UK homeland nor a deployed force could be protected from a concerted Russian air effort.”
The Army “has grown used to operating from safe bases in the middle of its operating area, against opponents who do not manoeuvre at scale”, he said.
Manpower in all three services was dangerously squeezed and navy ships and RAF planes have become used to depending on US support, he said.
“Key capabilities such as radars, fire control systems and missile stocks are deficient,” he said.
Sir Richard said: “There is a sense that modern conflict is ordained to be only as small and as short term as we want to afford, and that is absurd…
“The failure to come to terms with this will not matter at all if we are lucky in the way the world happens to turn out, but it could matter a very great deal if even a few of the risks now at large conspire against the UK.”
Sir Richard served as head of Joint Forces Command between 2013 and 2016, a role that saw him in charge of more than 20,000 military and civilian personnel across all three services.
BBC defence correspondent Jonathan Beale said it was not the first time a senior British military commander had spoken against defence cuts or what has been called the hollowing out of the armed forces.
But he said other criticism had not contained so much detail or been expressed as starkly.
Our correspondent said a defence source had questioned the motive behind the release of the memo, wondering if it was “sour grapes” as Sir Richard had been one of the candidates put forward for promotion as head of the armed forces.
A Ministry of Defence spokesman said: “Our defence review last year put in place a plan for more ships, planes and troops at readiness, alongside greater spending on cyber and special forces.
“That plan was backed by a rising defence budget. And, crucially, it was backed by all of the service chiefs, who were heavily involved putting it together.”