DAVID Cameron has slapped down Jeremy Corbyn and branded him naïve after the Labour leader questioned the allied attack that killed Jihadi John, the leading Islamic State(IS) terrorist, and spoke out against a shoot-to-kill policy in the event of a terror attack in the UK.

In a keynote speech at the annual Lord Mayor’s Banquet in the City of London, the Prime Minister also insisted Britain must show the same resolve it displayed against Hitler during the Blitz to defeat the threat of terrorism, using both hard and soft power.

And he announced plans to spend an additional £2 billion on the SAS and other special units over the coming five years to improve their fighting capabilities. It is understood the money will not be used on a recruitment drive but will fund new weapons and vehicles - possibly including helicopters - protective equipment, night-fighting kit and communications.

The additional cash will be delivered from the growth in defence budgets guaranteed by the UK Government's commitment to spend two per cent of GDP on the military.

Hours earlier, Mr Corbyn questioned the legality of last week’s attack, carried out by the US with support from the UK, which resulted in the death of British terrorist Mohammed Emwazi, known as Jihadi John.

In several videos, Emwazi taunted the West and beheaded a number of hostages on camera including two Britons, David Haines, a Scottish aid worker, and Alan Hemming, a Salford taxi driver, as well as the US journalist James Foley.

Mr Corbyn, stressing how he would only authorise actions that were “legal in the terms of international law," was asked if he believed the attack on Emwazi was legal. He replied: "I question that. Surely if somebody is doing something wrong, you act legally against them.”

The Labour leader was also pressed on whether or not he would be happy to order the police or the military to shoot to kill if there was a Paris-style attack on Britain's streets.

He replied: "I'm not happy with the shoot-to-kill policy in general; that is quite dangerous and can often can be counterproductive…The idea you end up with a war on the streets is not a good thing."

Asked if he could ever support military action against IS, Mr Corbyn said: “I'm not saying I would or I wouldn't; it's a hypothetical question at this stage.”

In his speech, Mr Cameron did not refer by name to the Labour leader but his remarks were clearly directed at him.

Stressing how confronting murderous violence required a strong security response, the PM said: “Those who say we should have somehow arrested Jihadi John, don’t get the reality of the world we are in...”

Referring to how British jihadists, who had been killed by drone strikes, had been planning to kill people in Britain, Mr Cameron stressed: “In that situation, you do not protect people by sitting around and wishing for another world; you have to act in this one and that means being prepared to use military force where necessary.”

In his speech, the PM confirmed the plan to increase Britain’s security and intelligence staff by an additional 1900 staff and to double to nearly £20 million spending on aviation security.

But he admitted that using military force and counter-terrorism expertise would only get Britain so far and what was needed was to address the root cause of extremism.

“This ideology, this diseased view of the world, has become an epidemic; infecting minds from the mosques of Mogadishu to the bedrooms of Birmingham. We have to stop it at the start; stop this seed of hatred even being planted in people’s minds, let alone allowing it to grow.

“That means confronting the ideology with our own liberal values and exposing this extremism for what it is: a belief system that glorifies violence and subjugates its people; not least Muslim people.”

Earlier in a Commons statement, Theresa May said the terrorists responsible for the massacre in Paris "represent no-one and they will fail" to divide free countries such as Britain and France.

The Home Secretary said she would be lobbying her counterparts across Europe for tighter gun laws and told MPs reviews of Friday night's attacks were under way to make sure any lessons would be learnt by the British police and security services.