Sir Keir Starmer has shown “corrosive complacency” towards defence, a key government adviser has warned, saying the UK’s security is “in peril” as a result.

Lord George Robertson, the former Nato secretary general who was appointed by the prime minister to write the government’s Strategic Defence Review (SDR), will use a speech on Tuesday to accuse “non-military experts” in the Treasury of “vandalism” and accuse Sir Keir of being unwilling “to make the necessary investment” in Britain’s defence.

Piling in on Lord Robertson’s criticisms, former defence secretary Malcolm Rifkind argued that the government’s “prime responsibility” is defence, calling for ministers to fund this through a cut to welfare or an increase in income tax.

The government has promised to publish a 10-year defence investment plan to fund the SDR, but it has been beset by repeat delays amid warnings that the military is facing a £28bn black hole in its funding over the next four years.

In a damning intervention, Lord Robertson – a Labour peer and former defence secretary – will also take aim at Sir Keir Starmer’s failure to tackle to the ballooning benefits bill, warning: “We cannot defend Britain with an ever-expanding welfare budget.”

The speech has come as a major shock partly because Lord Robertson has until now kept his advice and any criticisms strictly private and rarely speaks out.

Amid escalating turbulence in the Middle East and the ongoing conflict in Ukraine, Lord Robertson will say: “We are underprepared. We are underinsured. We are under attack. We are not safe . . . Britain’s national security and safety is in peril.”

According to the Financial Times, 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.”

Speaking to the BBC Radio 4’s Today programme, another one of the SDR’s authors General Sir Richard Barrons agreed with Lord Robertson that “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”.

He had earlier warned that the military is so depleted it could only “seize a small market town on a good day”.

Meanwhile, Sir Malcolm told The Independent he “fully agrees” with Lord Robertson.

“The government must decide what is its prime responsibility. It is the defence of the realm. That means they must increase the resources available to the Armed Forces paid for either by a reduction in the welfare services or an increase in Income Tax or VAT; or a combination of these sources”, he said.

“They must have the courage to explain this to the public. They will have to make this judgment at some time. The sooner the better.”

Meanwhile, another former defence secretary Penny Mordaunt told The Independent that Lord Robertson is “absolutely right”.

“The Treasury must realise that it can’t have growth if our data, energy, trade and interests aren’t protected”, she added.

The government has committed to spend 2.5 per cent of gross domestic product on defence by 2027, increasing to 3 per cent in the next parliament and a Nato-agreed target of 3.5 per cent by 2035.

Sir Keir told MPs on Monday that the government is working to finalise the defence investment plan but he did not want to repeat the mistakes of previous administrations because “we inherited plans that were unfunded and not deliverable”.

Lord Robertson’s intervention comes just days after former defence secretary Ben Wallace accused ministers of “taking the public for fools” after the current defence secretary John Healey insisted the UK is ready to defend itself.

“The problem is they simply will not make the tough political decisions… So we end up with very hollow platitudes and partisan comments from a Labour Party who think spin is the solution”, Sir Ben told The Independent over the weekend.

MPs on both sides of the Commons have already expressed concerns over the failure of the government to publish its Defence Investment Plan.

Meanwhile, in-year savings were blamed for a delay in sending HMS Dragon to Cyprus weeks after the Iran crisis began and the UK’s base on the island came under attack from Tehran.

There are also ongoing question marks over the reduction of the size of the UK army which is down to around 70,000, and using poor and outdated heavy vehicles and tanks.

A government spokesman 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 £270 billion being invested across this Parliament.

“We are finalising our defence investment plan that we will publish as soon as possible, putting the best kit and technology into the hands of our forces, rebuilding British industry to make defence an engine for growth and doubling down on our own commitment to Nato.”

Tory leader Kemi Badenoch said she “100 per cent” agreed with Lord Robertson’s damning criticism.

She said: “We used to spend one in every seven pounds on welfare. Now it’s one in every three pounds, and a lot of that money has basically been swapped for defence. The world is not as peaceful as it used to be. The peace dividend that existed after the fall of the Berlin Wall is gone. We need to spend more money on defence.

“This is a former Labour defence secretary, former NATO secretary general, saying what I’ve been saying for the past few weeks.”