The Hampshire-based Royal Navy's flagship aircraft carrier and its fleet of Harrier jets is to be scrapped ''with immediate effect'', Government sources said.

The move is among wide-ranging measures to be unveiled by Prime Minister David Cameron as part of the military's share of the Coalition's severe public spending squeeze.

He is also expected to confirm today that the Strategic Defence and Security Review (SDSR) has concluded that the renewal of the Trident nuclear deterrent will be delayed.

The immediate axeing of Portsmouth's HMS Ark Royal will leave the UK without an aircraft carrier capable of flying jets for around a decade while two new vessels are built at a cost of £5.2 billion.

That project was spared the axe after it was found abandoning the contract - with one of the two already under construction - would end up costing the taxpayer more.

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Although the first is set to come into service in 2016, converting it to allow it to be used by jets from allied nations could take several years.

Neither will be able to be used for British military jets until 2020 and one could yet be mothballed and possibly sold under future options still being looked at by the military.

Defence Secretary Liam Fox has insisted the long gap in Britain's aircraft carrier capability will not affect its defence abilities - despite warnings from former Navy chiefs.

But Mr Cameron was reported to have told the Cabinet it was one of the hardest decisions he had faced since becoming Prime Minister.

Under the SDSR, all three services face manpower cuts, the Army is expected to lose large numbers of tanks and the RAF to see bases closed and fighter jets axed.

Mr Cameron yesterday laid the ground for the reductions by signalling a shift in the UK's security priorities towards the threat of terrorism and cyber attacks.

Publishing a National Security Strategy (NSS), Mr Cameron said the present defence and security structure was ''woefully unsuitable for the world we live in today''.

A large-scale conventional military attack was ranked only in the third tier of threats to the UK which Labour said was being used as ''cover'' for a ''rushed'' defence review.

It will underpin moves towards mobile military units, intelligence-gathering and special forces and away from the tank brigades and jet fighters which dominated defence thinking in the Cold War.

Mr Cameron's personal intervention helped keep the cuts forced on the Ministry of Defence to around 8% rather than the 10% wanted by the Treasury as part of deficit-reduction efforts.