Keir Starmer has some big decisions to make - and may not have long to make them

Barely a day after lift-off for the Strategic Defence Review, there are serious questions about whether the government needs to go further.

It has emerged senior defence figures believe the prime minister will have to bow to pressure from NATO to announce a target of spending 3.5% of GDP within a decade.

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It's no surprise the 32-member defence alliance - facing its greatest twin challenge since the end of the Cold War - is calling for more money.

With Russia bringing war to Europe, and Donald Trump making clear the US is not willing to fund its security indefinitely, countries are already stepping up like never before.

What is stark is the widespread acknowledgement in Whitehall that spending 3%, which is still only a government ambition for the next parliament, will be a minimum.

The authors of the Strategic Defence Review make clear Britain is most likely to fight a future war not on its own, but as a leader in NATO - and NATO's spending ambitions are soaring.

Money is tight - and there are plenty of priorities

The secretary general of NATO, Mark Rutte, is calling for 3.5% on hardware and an overall 5% target for military, cyber and intelligence spending.

Keir Starmer's spokesman insisted today it is "not just about cash" - the UK steps up as a member with its nuclear programme, combat aircraft, carriers, and the quality of its personnel.

But with only a firm commitment to a rise to 2.5% in two years' time, and the Ministry of Defence already over budget on its current commitments, the ambitions set out for 12 new attack submarines, F35 aircraft carrying nuclear warheads and a home guard will require further injections of cash.

The government's reluctance to go further at this stage is understandable. The rise to 2.5%, announced by the prime minister in February, required a controversial cut to the foreign aid budget - and the resignation of a cabinet minister.

Welfare reforms are causing Labour MPs further pain. As one put it to me: "The party doesn't, in the main, oppose any of the defence decisions. The fault line will be that if there's money for that, why can't there be money for public services."

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Are tax rises inevitable?

The price tag of a rise to 3.5%, from 2.5%, would be £30bn a year. Paul Johnson, of the Institute for Fiscal Studies, said "chunky" tax rises would be required.

"We could get away with not having tax increases if the economy starts growing much, much faster than it has over the last 15 years," he told Sky News. "But if it doesn't and the government can't think of other bits to stop or cut then inevitably we'll have tax rises."

'We have to up our game'

Tan Dhesi, chair of the Commons defence committee, who visited the US and spoke to Pentagon and State Department officials last week, said the UK had to "up its game" - but also its messaging.

"We've got growing concerns that the wider public may not be aware of," he said.

"Firstly, we've got the US. Their stated intention is to focus more elsewhere, on homeland security and the Pacific. They want the Europeans effectively to fund more for themselves. So, we collectively have to increase our focus on spending on defence.

"We've also got the fact that Russia in Ukraine and spending more than 40% of its budget on defence. They're rearming at a rate of knots. Likewise, 30% with China.

"The UK is also the third most targeted nation on the planet in terms of cyberattacks. For all those reasons, we have to up our game."

He said the push to 3.5% was "inexorable", and the prime minister then has "a very difficult decision to make."

Polling by YouGov on Monday showed public support for making cuts in other areas to fund defence spending hikes was 29%, with 53% opposed.

The prime minister often says the first duty of his government is to keep people safe. It's particularly acute for him, because of the legacy of Jeremy Corbyn's leadership, and because Mr Trump and his push for a ceasefire in Ukraine, which Europe would police, has changed the calculations.

But hiking defence spending further will require taking the public with him - and he may not have long to start that work.