Deep cuts to the defence budget will see the Royal Navy's flagship aircraft carrier and its fleet of Harrier jets scrapped “with immediate effect”, sources said.
The move is among wide-ranging measures due to be unveiled today by Prime Minister David Cameron as part of the military's share of the Coalition's severe public spending squeeze.
He was 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 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 cost taxpayers more.
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 years.
Neither will be able to be used for British military jets until 2020 and one could yet be mothballed and possibly sold.
Defence Secretary Liam Fox insists the long gap in Britain's aircraft carrier capability will not affect its defence abilities.
But Mr Cameron reportedly told the Cabinet it was one of the hardest decisions he had faced since becoming Prime Minister.
Mr Cameron yesterday laid the ground for the reductions by sig
nalling 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”.
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.
Mr Fox has conceded that personnel numbers will “inevitably... fall a bit” but refused to discuss reports the forces could lose a combined total of as many as 20,000.
Some reports have put the figure for the Army at 7,000 although it is predicted to escape the most serious cuts at least while operations continue in Afghanistan.