Labour was branded the 'Welfare Party' on Wednesday after Andy Burnham said he would never prioritise defence spending over benefits.

In an extraordinary intervention, the Prime Minister said he was serious about national security, but it 'cannot come at the expense of social security'.

Writing in the Daily Mail, Tory leader Kemi Badenoch said Mr Burnham had 'said the quiet part out loud', adding: 'For our new PM, social security – welfare – comes above national security.

'The Labour Party has become the Welfare Party. The first duty of government is defence of the country, not paying more benefits.'

Last week, Mrs Badenoch laid out detailed plans to find £10billion to raise defence spending to 3 per cent of GDP by 2030. The package included £8billion of cuts to the welfare budget, half of it through reforms to the runaway housing benefit bill.

During angry clashes in the Commons during Prime Minister's Questions, Mr Burnham refused to commit to the 3 per cent target, which his new Chancellor John Healey has warned is the minimum needed to keep Britain safe.

And the PM rounded on the Conservatives for proposing 'crude' benefit cuts to fund the Armed Forces. 'We do everything to support our national security, but it cannot come at the expense of social security,' he said.

Defending his support for higher welfare spending, he said strong national security 'also depends on resilience in our communities'. He said Labour would never be 'forced to choose between the two'.

The PM's comments create a clear dividing line in British politics over how quickly defence spending should increase in the face of rising threats – and how it should be paid for.

They also appear to be at odds with remarks from new Defence Secretary Wes Streeting, who said in June that 'there is no social justice without national security'. And they will raise fresh doubts about whether Labour will ever act to rein in the runaway welfare bill.

Earlier this year, messages released in the Mandelson files revealed that work and pensions secretary Pat McFadden told the disgraced peer that Labour MPs were demanding more welfare spending, saying: 'Every meeting I have is: 'Who can we tax in order to pay benefits to others?'

Tory welfare spokesman Helen Whately said: 'Does Burnham know what happens when a vulnerable country gets attacked? Without national security, there will be no social security.' Fellow Tory Claire Coutinho said the PM's comments were 'barking mad'.

Reform UK's deputy leader Richard Tice accused Mr Burnham of 'surrender', adding: 'He's basically saying mild anxiety is more important than the safety of the realm.'

Spending on sickness and disability benefits is on course to hit £109billion by the end of the decade – an increase of more than £30billion on last year. Sir Keir was forced to abandon plans to trim £5billion from the welfare bill last year following a mutiny by Labour MPs.

Mr Burnham on Wednesday confirmed he was opposed to any 'crude cuts' to benefits. He claimed that long-term reforms to housing and education could eventually bring the benefits bill down.

But one Treasury insider told the Mail that the welfare budget is now the only potential source of significant revenue: 'In the past, Chancellors could always find a few billion pounds down the back of the sofa if they really had to. 

'But all the low-hanging fruit is gone. We can't borrow more, we can't really tax much more – the only thing left is welfare.'

Britain has suffered an alarming slide down the Nato spending league table in recent years as other countries ramp up investment to meet growing threats. After being a top player for decades, the UK is now in 13th place in terms of defence spending as a percentage of GDP.

Under Labour's plans, spending will nudge up from 2.6 per cent next year to 2.7 per cent by the end of the decade. By contrast, Germany expects to spend 3.5 per cent by 2030 while Poland is already spending 4.7 per cent.

Mr Burnham is under pressure from MPs on all sides to increase defence spending in the face of growing threats from Russia and other hostile states.

Tan Dhesi, Labour chairman of the Commons defence committee, last week accused him of 'kicking the can down the road'. In his resignation as defence secretary in June, Mr Healey said Labour's plans 'fall well short of what is needed for defence and the country at this dangerous time', and urged ministers to move to 3 per cent by 2030.

But neither he nor Mr Burnham are willing to commit to hitting the target by the end of the decade. Instead, Mr Burnham on Wednesday adopted the looser target of spending 3.5 per cent by 2035.

The Tory plans involve saving at least £4billion a year through reforms to housing benefit, and restoring the two-child benefit cap, which Labour scrapped this year, raising a further £3.1billion.

Another £1billion would be saved by ending VAT exemptions for Motability vehicles. The remainder of the £10billion would be funded by scrapping the controversial Chagos Islands giveaway and by diverting money from spending on new windfarms and civil research and development.

Downing Street insisted Mr Burnham was committed to bringing down the welfare bill but refused to put a timescale on it.

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