The UK’s armed forces would not be able to protect the country from a full-scale attack by Russia or another major military power, the recently retired commander of joint forces command has said.

Gen Sir Richard Barrons, who stepped down in April, delivered a scathing assessment of the UK military in a 10-page private memorandum to the defence secretary, Sir Michael Fallon.

It comes despite the government’s decision to increase defence spending by nearly £5bn by 2020/21 and to meet Nato’s target of 2% of GDP for the rest of the decade.

In his memo, Barrons said: “Capability that is foundational to all major armed forces has been withered by design.

“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.”

The document, seen by the Financial Times, gives a withering judgment of Britain’s ability to defend itself against a full-scale military attack and singles out Russia, a country seen as more dangerous and unpredictable since its annexation of the Crimea and incursion in Ukraine.

Barrons said: “Counter-terrorism is the limit of up-to-date plans and preparations to secure our airspace, waters and territory ... there is no top-to-bottom command and control mechanism, preparation or training in place for the UK armed forces [to defend home territory] ... let alone to do so with Nato.”

On Britain’s ability to defend itself from aerial attack, he said: “UK air defence now consists of the [working] Type 45 [destroyers], enough ground-based air defence to protect roughly Whitehall only, and RAF fast jets.

“Neither the UK homeland nor a deployed force – let alone both concurrently – could be protected from a concerted Russian air effort.”

Barrons said the army’s recent experience did not include conducting full-scale wars, which could also be a disadvantage.

“The current army has grown used to operating from safe bases in the middle of its operating area, against opponents who do not manoeuvre at scale, have no protected mobility, no air defence, no substantial artillery, no electronic warfare capability, nor – especially – an air force or recourse to conventional ballistic or cruise missiles,” he said.

Barrons also raised concerns that Britain’s armed forces were dominated by a small number of expensive pieces of equipment such as new aircraft carriers which “we cannot afford to use fully, damage or lose”, and he bemoaned a lack of manpower across the military.

Maj Gen Tim Cross, who served in the army for nearly 40 years, rejected suggestions that Barrons’ comments were a case of sour grapes because he was overlooked for promotion.

He said Barrons was an “extremely capable operator”.

“Like all commanders, and I did this myself when I handed over command of my division, what Richard has done is produce what we call a haul down report,” he told BBC Radio 4’s Today programme.

“A haul down report is a state of the nation, it’s a final shot to say to the system - in this case the secretary of state because Richard was a four-star general - ‘This is my command, this is what it looks like, these are my concerns,’ and he’s laid those out. He’s speaking truth to power and it’s a normal thing to be doing.”

Fallon, meanwhile, has told the Times that Britain will veto measures to build an EU army for as long as it remains a member of the union. His comments came as it emerged that France and Germany had drawn up a timetable to create a “common military force” that would rival Nato in capability.

According to a document discussed by EU leaders at a summit in Bratislava on Friday, the European commission will put forward proposals in December for such a force.

“That is not going to happen,” Fallon said. “We are full members of the EU and we will go on resisting any attempt to set up a rival to Nato.”

The former Liberal Democrat leader Menzies Campbell said there was nothing the UK could do after Brexit to protect Nato from the potentially damaging effect of an EU army.

The peer, who is a member of the UK parliamentary delegation to the Nato assembly, said: “Even as a fervent European, I regard the creation of a European army as a deeply damaging, long-term threat to Nato.

“The cornerstone of European defence is Nato, of which the United States is the most senior partner contributing 75% of the budget of the alliance. The creation of a European army will only encourage isolationists in the United States to argue that Europe should be responsible for its own defence.”