The Ministry of Defence is bracing for what one Whitehall official has described as “brutal” cuts in a security review scheduled to be announced early next year.

The scale of proposed cutbacks in the army, navy and air force has created alarm within the MoD and poses a major headache for the new defence secretary, Gavin Williamson.

The Times reported that Tobias Ellwood, the minister in charge of personnel and veterans and a lieutenant-colonel in the army reserve, is threatening to resign if the army, which has already been reduced to 82,000, is cut back further.

The Cabinet Office review is looking at 12 different strands of UK security including the intelligence services MI5 and MI6 and the surveillance agency GCHQ, as well as the MoD. Against a background of increased terrorist attacks this year, the intelligence agencies appear to be escaping lightly while the armed services take the brunt of the cuts.

The MoD has drawn up a list of options for cuts, resultingfrom a budget squeeze and a planned shift to projects judged to be more relevant to modern warfare. One of the key planned changes is to devote more resources to cyberwarfare given the threat posed by hackers to essential supplies and to an increased use of drones, robots and artificial intelligence.

The MoD is also looking at ways to better combat the kind of hybrid warfare that Russia has engaged in in eastern Ukraine, a mixture of irregular forces and psychological operations.

Areas vulnerable to cuts are the marines and amphibious vessels. Other budget-saving measures could include delays in building frigates or reducing the number of F-35s bought for two new aircraft carriers. An ambitious army plan to be able to deploy a new 30,000-strong division by 2025 could be in jeopardy.

The two new aircraft carriers, regarded by some within the MoD as vanity projects, would have been high on the list for axing at an earlier stage. The replacement for Trident nuclear programme, including our new submarines, is one of the costliest parts of the £36bn defence budget but is viewed by the MoD as untouchable.

The Cabinet Office exercise began as a mini-review that has since turned into a major defence review even though the last one was only completed in 2015.

Recently-retired senior officers, giving evidence earlier this month to the Commons defence committee, expressed concern that the armed forces are no longer fit for purpose and the army is 20 years out of date.

Gen Sir Richard Barrons, who retired last year as commander of joint forces command, said the armed forces were close to breaking point, with the navy underfunded and the air force at the edge of its engineering capacity.

The review is being headed by Mark Sedwill, a diplomat who served in Afghanistan and is now the national security adviser at the Cabinet Office.