With his government unveiling the update to Britain's national security and international policy, Sunak, on a visit to the US, will also set out an "ambition" to increase defence spending to 2.5 per cent of gross domestic product in the longer term.
Sunak hailed the move as a way "to ensure we are never again vulnerable to the actions of a hostile power", but his offer of five billion pounds is less than half of what some in his governing Conservative Party say is needed to be able to support Ukraine against Russia, while not leaving Britain vulnerable.
He said his previous increases in defence spending showed he was a man of his word, and described the new commitments as a "strong and positive statement".
"As the world becomes more volatile and competition between the states becomes more intense, the UK must be ready to stand our ground," he said in a statement.
"We will fortify our national defences, from economic security to technology supply chains and intelligence expertise, to ensure we are never again vulnerable to the actions of a hostile power."
The Ministry of Defence said minister Ben Wallace was "delighted" with the settlement, especially in the tough economic times, and said it would maintain the upward trajectory after the government invested heavily in recent years.
The unveiling of the updated Integrated Review has been choreographed to coincide with Sunak's visit to San Diego to agree on the next steps in the landmark AUKUS defence agreement with the US and Australia.
Sunak, who will meet with US President Joe Biden and Australian Prime Minister Anthony Albanese, will want to underline that the increase in spending will only bolster the AUKUS pact.
Some of the new spending will go towards programs that will help Australia build nuclear-powered submarines for the first time, part of efforts to counter China in the Indo-Pacific.
"As I will discuss with our American and Australian allies in the US today, the UK will remain a leading contributor to NATO and a reliable international partner, standing up for our values from Ukraine to the South China Sea," he said in the statement.
But Sunak is under pressure at home to offer more help to the defence ministry to combat the impact of inflation and spur production of ammunition and other military hardware to replace weapons sent to Ukraine to help Kyiv push back Russian forces.
Britain and other Western countries have scaled up their pledges of military aid for Ukraine this year, with promises of tanks and armoured vehicles, as well as longer-range weapons.Â
London has also offered to train Ukrainian soldiers on warplanes rather than delivering fighter jets as yet.
While Sunak's foreign minister, James Cleverly, is due to unveil the updated strategy, the British leader will hope to set the tone in San Diego, saying the "refresh" will set out how Britain has adapted its approach to China.
Sunak said on Sunday China presented an "epoch-defining challenge" to the global order but it would not be a "smart or sophisticated foreign policy to reduce" the relationship with Beijing to just two words, such as labelling it "a threat".
