Sir Keir Starmer is under pressure to quickly lift defence spending to 2.5% of GDP or face new military cuts this year – even as he considers deploying troops to Ukraine, Sky News understands.

Defence sources said such an increase – which would amount to about an extra £5bn annually – is still far short of what is required to rebuild and transform the armed forces, stressing that an ultimate hike to at least 3% of national income would be necessary.

But the sources said a rapid rise in investment to the government’s promised target of 2.5% of GDP, from 2.3% at present, should prevent new swingeing reductions in capabilities – just as Donald Trump orders European militaries to be stronger.

“The truth is there needs to be more money now or else prepare for further cuts,” said one military insider, who spoke on condition of anonymity.

A second defence source said: “We know the government is in a difficult financial position. But getting to 2.5% sooner rather than later would be enormously beneficial for their relationship with the new US administration, and the UK’s leadership role within NATO.”

The prime minister has pledged to set a path to investing 2.5% of GDP on defence but he has yet to say when this commitment will be announced, let alone by what date the target will be met.

Defence sources said they believe the Treasury wants to push the timeline out to the very end of this parliament – a delay that would leave the armed forces to “wither on the vine”.

“It is just not serious,” said a third defence source, sounding exasperated.

The UK position on defence spending came into sharp focus last week as European allies reeled from a barrage of criticism by the new Trump administration for their over-reliance on the US to defend Europe and support Ukraine.

Britain – a nuclear power – presents itself as the strongest European military within the NATO alliance and boasts of a special relationship with the US.

Yet, Sir Keir has stuck with his 2.5% target even as Mr Trump calls on allies to boost defence expenditure to 5% and take on a much greater responsibility for security in Europe.

Mark Rutte, the secretary general of NATO, says the goal for all 32 allies should be “north of 3%”.

The reality of Europe’s weakness on defence has been brutally exposed by Mr Trump’s approach to ending Russia’s war in Ukraine, with the president sidelining the UK, the Europeans and Kyiv.

US envoys are due to meet Russian officials in Saudi Arabia on Monday as France’s president hastily convenes a meeting of European leaders, including Sir Keir, in Paris.

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But, given the hollowed-out state of the UK armed forces, an announcement by the prime minister that he was considering sending British soldiers to Ukraine to help secure the peace as part of any ceasefire deal raised eyebrows within defence circles.

One insider questioned how Mr Starmer could propose such a challenging deployment without explaining how he was going to fill the army’s gaping gaps in weapons and manpower.

Ukrainian President Volodymyr Zelenskyy has said any international security force of British and other foreign troops would need to be about 110,000-strong.

The UK would struggle to sustain a deployment of several thousand soldiers for any enduring length of time and would have to give up other commitments, such as a battlegroup of around 900 military personnel based in Estonia on a NATO mission to deter Russian aggression.

With defence matters in focus, Admiral Sir Tony Radakin, the head of the armed forces, and his fellow chiefs met with Sir Keir on Friday to talk to him about military capabilities.

It is highly unusual for such a meeting to take place and signals a desire by the prime minister to understand the thinking of his top brass.

The meeting came as an external team of experts, led by Lord Robertson, a former Labour defence secretary and former secretary general of NATO, was putting the finishing touches to a sweeping review of defence that the government has said will be published this spring.

The reviewers were tasked with setting out how to transform the army, Royal Navy and Royal Air Force to meet future threats, but their ability to deliver has been constrained from the start by Downing Street saying this must be achieved within a defence spending target of 2.5% of GDP.

A government spokesperson said any suggestion the prime minister was considering raising defence spending beyond 2.5% “is purely speculation”.

The spokesperson said: “The Strategic Defence Review is wide-ranging, ensuring we look hard at the threats we face and the capabilities we need to meet the challenges and opportunities of the 21st century. As we have consistently said, the review will be published in the spring.

“To ensure the UK is prepared to deal with the changing threat, our budget increased defence spending by £2.9bn for next year and we are committed to setting a path to 2.5% of GDP on defence.”