The Government is pumping £1.1 billion into drones and other high-tech surveillance kit to tackle "unseen enemies", David Cameron has said.

The Prime Minister said increasing capability was a "national necessity", arguing that Britain could not "defend the realm from the white cliffs of Dover".

The announcement, which Mr Cameron insisted was possible due to the success of austerity measures, came as he visits the Farnborough Air Show in Hampshire.

The funding pot includes an extra £800 million of investment in an intelligence, surveillance, target acquisition and reconnaissance package.

It will bolster special forces' ability to deal with the threat of global terrorism and hostage taking, according to 10 Downing Street.

A further £300 million will be used for existing programmes, including unmanned aircraft and next generation radars for Typhoon combat jets.

Writing in The Daily Telegraph, Mr Cameron insisted: "Today's investment demonstrates our approach to national security. There are those who believe we would be safer if we fundamentally retreated from the world.

"They see new warships and military investment and imagine a Government bent on foreign adventurism.

"But the plain fact is that in the 21st century, you cannot defend the realm from the white cliffs of Dover. Terrorist plots hatched thousands of miles away threaten to cause harm on our streets. When fragile and lawless states fracture, migration flows can affect us right here."

Touring the show today, Mr Cameron will also outline plans to set up a UK Defence Solutions Centre in Farnborough to develop new defence technology.

A £4 million UK Centre for Maritime Intelligent Systems based in Portsmouth is also being launched as well as a "defence apprenticeship trailblazer" scheme to attract new graduates to the industry as well as develop a new masters level standard in advanced systems engineering.

Mr Cameron will say: "Having modern, technologically advanced and flexible Armed Forces to protect us and our interests is vital.

"Because of the difficult decisions we have taken to tackle the deficit we are able to make these vital investments in our defence capabilities.

"We are also taking action to sustain our thriving defence industry, as part of our long-term economic plan to back business, create jobs and secure a brighter future for hard-working people."

Mr Cameron had been due to see the new F-35 fighter aircraft at Farnborough today - but embarrassingly it has still not been given clearance to fly by the Americans following an engine fire in Florida last month.

The grounding meant the jet was also unable to appear when the Queen named the Royal Navy's new aircraft carrier Queen Elizabeth earlier this month, and at a military tattoo at RAF Fairford in Gloucestershire this weekend.

"The aircraft is still awaiting US clearance but we are hopeful that it will fly at the air show by the end of the week," a spokesman for Farnborough airshow said.

"Everyone involved in the project is working towards a positive result for attendance at the air show this week, and we fully support the stance to never compromise safety of either pilots or show participants and we thank them all for their continued hard work."