British troops training
Gen Sir Richard Barrons said the UK is ill-prepared to defend against a serious military attack Getty Images

The UK's armed forces are ill-prepared for a military attack, according to a senior commander of the British Armed Forces.

Gen Sir Richard Barrons issued a memo before his retirement in April as head of Joint Forces Command warning that key capabilities had been sidelined to cut costs.

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According to the Financial Times, which obtained the 10-page private memo, he warned Whitehall was merely "preserving the shop window" with items like aircraft carriers.

However, Ministry of Defence officials said Sir Richard had backed the last defence review.

The memo was sent to Defence Secretary Michael Fallon, following the government's decision to increase defence spending by nearly £5bn ($6.5bn) by 2020-21, reaching Nato's target to spend 2% of GDP on defence until 2020.

Sir Richard said: "Capability that is foundational to all major armed forces has been withered by design.

"Counter-terrorism is the limit of up-to-date plans and preparations to secure our airspace, waters and territory.

"Neither the UK homeland nor a deployed force could be protected from a concerted Russian air effort."

He continued saying 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 also warned that crucial manpower in all three armed services was "dangerously squeezed" and often dependent on US support.

He 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."

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."

Sir Richard served as head of Joint Forces Command between 2013 and 2016 where he was in charge of more than 20,000 personnel across all three services.