Security minister Tom Tugendhat and Foreign Office minister Anne-Marie Trevelyan say the UK must “lead the way” amid growing threats to Britain.

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They piled pressure on Chancellor Jeremy Hunt in a 1,100 word joint essay on LinkedIn just two days after the Budget failed to boost defence spending.

And called for spending to be hiked at least 2.5 per cent of gross domestic product – the nation’s wealth.  It currently stands at around two per cent of GDP.

Whitehall insiders said the ministers had “gone rogue” with the post that had not been cleared with Downing Street first.

Despite the astonishing act of defiance by two minsters – neither are expected to be sacked.

“The sad truth is that the world is no longer benign,” they wrote.

“Protecting ourselves requires investment. And effective investment means that our industrial complex must grow and strengthen at much greater pace than at present.

“We cannot turn on the complex platforms and weapons which ensure military advantage overnight. We must start that growth now, invest at pace to support our allies and stay ahead of our adversaries.”

Former soldier Mr Tugendhat served in both Iraq and Afghanistan and attends Cabinet.

Ms Trevelyan is his close ally and backed his campaign to be Tory leader in 2022.

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In their letter, the pair pointed to China and Russia’s soaring defence spending: “It’s clear to us that the UK needs to lead the way in increasing our own domestic defence and security spending commitments to 2.5 per cent and beyond.

“Former defence secretary Ben Wallace and prime minister Boris Johnson made inroads into growing our defence budgets, which had been shrinking in real terms for years, but that only filled the hole. Now we need growth.”

Mr Hunt has said “spending will rise to 2.5 per cent as soon as economic conditions allow”.

However, senior military figures and former defence secretaries have been calling for more cash for the Armed Forces now.

Tory MPs Sources said even Defence Secretary Grant Shapps had been surprised there was not more in the Budget.

Mr Shapps wrote to the Chancellor in January formally requesting more defence spending.

Downing Street sources said the article did not need to be cleared because it is a social media post and it is Government policy to raise spending to 2.5 per cent as soon as possible.