David Cameron will pledge an extra £12 billion ($18.2 billion, 17.1 billion euros) for the military Monday as he pushes Britain's parliament towards a vote on joining air strikes in Syria within days.
The Strategic Defence and Security Review (SDSR) will map out Britain's defence strategy for the next five years after this month's Paris attacks which killed 130 people showed the Islamic State (IS) group's ability to strike in the heart of Europe.
Cameron will announce measures including the creation of two new 5,000-strong rapid reaction strike brigades which will be available for global deployment at short notice by 2025, Downing Street said.
"As the murders on the streets of Paris reminded us so starkly, Islamic State is not some remote problem thousands of miles away; it is a direct threat to our security," the prime minister wrote in the Daily Telegraph newspaper Monday.
"I want the British people to know they have a government that understands the importance of our national security and that we will take whatever actions are necessary to keep our country safe."
After talks with French President Francois Hollande in Paris Monday, Cameron said it was his "firm conviction" that Britain should join France and others in striking IS in Syria.
Hollande said Britain and France must work together to fight the jihadists, adding: "We have joint obligations".
- 'All necessary measures' -
Cameron will outline a "comprehensive strategy" for Britain joining action against IS in parliament later this week.
Reports suggest the government could then call a vote on the issue by the end of next week if ministers are confident of winning it.
While British forces are taking part in air strikes on IS targets in Iraq, they are not involved in the international effort targeting Syria due to resistance from opposition parties still mindful of Britain's unpopular interventions in Iraq and Afghanistan.
Jeremy Corbyn, leader of the main opposition Labour party, is against any military action but Cameron is increasingly confident in the wake of the Paris attacks that he can persuade enough Labour MPs to pass it.
He is also hopeful that last week's United Nations Security Council resolution authorising countries to "take all necessary measures" against IS will help make his case.
A Times/YouGov opinion poll last week found that 58 percent of people would approve of Britain joining air strikes on targets in Syria compared to 22 percent who were against it.
- Questions on police funding -
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The SDSR has been planned for months and will also include a 10-year extension to the lifespan of the Royal Air Force's Typhoon fighter jets, which are also being upgraded to give them ground attack capabilities.
The government will also announce that counter-terrorism funding is being increased by 30 percent in Wednesday's autumn statement delivered by finance minister George Osborne.
However, Osborne faces a row over claims from police chiefs that cuts to the number of frontline officers who do not fall under the counter-terrorism budget could increase the risk of an attack in Britain.
The SDSR is underpinned by the government's commitment to meet NATO's defence spending target of at least two percent of gross domestic product.
That came following criticism from the armed forces after the defence budget was reduced by some eight percent since 2010 in austerity cuts designed to tackle Britain's deficit.
While IS is currently top of the list of worries for the security services, parliament's Defence Select Committee said in a report Saturday that the government's approach to mapping the nature of future threats was "flawed".
"Greater emphasis should be laid upon military flexibility: the ability of versatile armed forces to cope with what cannot reliably be foretold," it said.