The Prime Minister has been warned this is “no time for dither” and he must announce an urgent increase in defence spending ahead of crisis talks about Ukraine with US President Donald Trump on Thursday.

Conservative leader Kemi Badenoch has offered Tory support for using cash from the welfare and foreign aid budgets to boost spending on Britain’s armed forces.

She warns in a letter to the PM that “Putin’s Russia poses an existential threat to the entire European order”.

Mrs Badenoch says spending should increase to 2.5% of GDP as a “bare minimum”.

She writes: “We should have spent recent months increasing our domestic defence production. Procuring munitions, equipment or weapons takes time.

“There is no time for further delay. I am willing to offer my party’s support if you need to make difficult decisions because the first priority of every Government must be our national security.”

Warning “this is no time for dither” and “these decisions cannot be delayed,” she says he should consider “whether some of the 0.5% of GDP currently spent on overseas development should be repurposed” to support defence and security.

She also suggests that savings made on “restrictions to welfare” could be used to boost defence spending.

Mrs Badenoch insists he should not wait until Thursday’s meeting with Mr Trump to set out plans for strengthening the armed forces.

The Tory leader also urges Sir Keir to abandon his “Government’s indefensible surrender of British territory in the Chagos Islands,” stating: “You should not be concluding a deal which would mean British taxpayers paying Mauritius billions of pounds to take our territory.”

She warns: “I will always back you when you do the right thing. But I will not be silent about the risks facing our country and our security.

“History has been unforgiving to politicians who, faced with growing threats, chose procrastination over action.”

Sir Keir’s meeting with Mr Trump comes as the transatlantic alliance faces unprecedented uncertainty. There are expectations Sir Keir will use the visit to say that UK defence spending will hit 2.5% of GDP in 2030.

Mr Trump shocked allies when he described Volodymyr Zelensky as a “dictator” but Sir Keir has told the Ukrainian leader his country is on an “irreversible” path to NATO membership.

YouGov polling show 62% of Britons support Ukrainian membership of NATO – but they doubt US commitment to the alliance. Nearly six out of 10 (58%) think the US would be “unlikely” to defend the Baltic states if Russia invaded and just 44% think America would come to Britain’s aid if the UK is attacked.

In a bid to strengthen relations between the UK and the US, Sir Keir may extend an invitation from the King for a state visit to the UK.

Oleksii Goncharenko, a Ukrainian MP from Odessa, was in no doubt about the message he wanted Sir Keir to deliver in his meeting with the US President.

“It would be a catastrophe to show that Putin is stronger than Trump or Putin is stronger than the free world,” he said, adding: “Ukraine cannot be abandoned and Putin shouldn’t be emboldened.

“That is the message – Ukraine is too important to lose.”

Monday marks the third anniversary of Russia’s invasion of Ukraine. Foreign Secretary David Lammy said the time had come to “turn the screws on Putin’s Russia”.

He said: “On the battlefield we remain committed to providing £3billion of military support a year to put Ukraine in the strongest position possible, and being ready and willing to provide UK troops as part of peacekeeping forces if necessary.”

Mr Lammy also pledged to introduce “largest package of sanctions against Russia since the early days of the war – eroding their military machine and reducing revenues fuelling the fires of destruction in Ukraine”.

Sir Keir spoke with Mr Zelensky on Saturday morning.

A Downing Street spokesperson said: “The Prime Minister repeated that Ukraine must be at the heart of any negotiations to end the war and that safeguarding Ukraine’s sovereignty was essential to deter future aggression from Russia.”