The government has shown a “corrosive complacency towards defence” and put the UK in peril, according to a government adviser, in fierce criticism of Keir Starmer’s military policy.

George Robertson, the former Nato secretary general and author of the government’s strategic defence review, believes Starmer is “not willing to make the necessary investment”, the Financial Times reported.

In addition, Lord Robertson will warn in a lecture in Salisbury on Tuesday that the Iran war “has to be a rude wake-up call”.

The former general Richard Barrons, who co-authored the defence review with Robertson, echoed his concerns. “It is a mark of how serious it is that someone who has been a Labour party activist for more than 60 years and was a Nato secretary general has now had to say it in these terms today,” Barrons told BBC Radio 4’s Today programme.

Robertson, a former defence secretary who led Nato from 1999 to 2003, will also accuse “non-military experts in the Treasury” of “vandalism”. “We cannot defend Britain with an ever-expanding welfare budget,” he said in an interview with the Financial Times.

He will say in his speech: “We are underprepared. We are underinsured. We are under attack. We are not safe … Britain’s national security and safety is in peril.”

Barrons said: “There’s an enormous gap between where we have to be to keep the country safe in the world we now live in, and where we actually are.”

Asked how he responded to Pete Hegseth, the US defence secretary, mocking the Royal Navy last week, Barrons said: “I hung my head in sorrow, but I couldn’t argue with him because although the Royal Navy and the Royal Air Force and the army are, in their bones, outstanding institutions, they are simply too small and too undernourished to deal with the world that we now live in. And the review says this.”

The government’s proposals to fund the strategic defence review recommendations, including a 10-year defence investment plan due by last autumn, have been repeatedly postponed amid warnings that the military faces a £28bn funding gap over the next four years.

Barrons said: “The choice on the prime minister’s desk is they either find some more money to implement a new de minimis review at the speed we agreed last year, or he is going to announce £28bn worth of cuts. And how would that fit with the world that we find ourselves in today?”

The Ministry of Defence, the Treasury and Downing Street have not reached an agreement about how to proceed, according to sources.

Robertson believes spending cuts in other departments may be required to boost defence funding. In his speech, he will say the chancellor, Rachel Reeves, “used a mere 40 words on defence in over an hour” in her budget speech last year, and last month “in the spring statement she used none”.

He will add: “There is a corrosive complacency today in Britain’s political leadership. Lip service is paid to the risks, the threats, the bright red signals of danger – but even a promised national conversation about defence can’t be started.”

In February Luke Pollard, the minister for defence readiness and industry, told the Guardian that the investment plan was “a bigger task than many people outside defence realise”.

It would mean “fundamentally changing the shape of our armed forces, so pivoting, in particular, towards more autonomy”, he said, while also stressing the need to refill military stockpiles sent to Ukraine in recent years. “It is not a simple matter of just replacing tank A with tank B.”

Robertson said he would cite the country’s inability to deploy more than one Royal Navy warship to the Mediterranean within the first fortnight of the Iran war to illustrate the UK’s complacency towards defence.

In the speech, he will warn that the UK faces not just shortages of military kit but “crises in logistics, engineering, cyber, ammunition, training and medical resources”.

Last week the defence secretary, John Healey, exposed a covert Russian submarine operation targeting critical undersea infrastructure around UK waters.

A government spokesperson said: “We are delivering on the strategic defence review to meet the threats we face. It is backed by the largest sustained increase in defence spending since the cold war, with a total of over £270bn being invested across this parliament.”

They said the government was finalising the defence investment plan and would publish it as soon as possible.