Government defence cuts that will prompt hundreds of Westcountry personnel to leave the service have been heavily criticised in two hard-hitting reports.

The Whitehall spending watchdog, the National Audit Office, will today warn the armed forces risk losing vital skills because of the speed of Ministry of Defence cost-cutting.

The analysis comes after MPs on the Commons Defence Select Committee argued military engagements on the scale of the intervention in Libya would be difficult to repeat.

Devonport Naval Base, home of warships and submarines sent to the Mediterranean last summer, will bear the brunt of 5,000 sailors, submariners, pilots and officers who will depart the senior service by 2015. It will leave Britain with a standing maritime force of around 30,000.

The MoD is in the process of cutting 25,000 armed forces personnel and 29,000 civilian staff by 2015 in the biggest round of cuts to the military since the end of the Cold War.

The reductions are intended to reduce costs by £4.1 billion as part of the drive to plug a £38 billion "black hole" in the defence budget.

The NAO said the department was being forced to push through reductions to military and civilian personnel before it had drawn up detailed plans as to how it would operate in future. It warned the MoD faced "profound changes" if it was to continue meeting its commitments with fewer people.

The head of the NAO, Amyas Morse, said: "The department has acted decisively, but runs the risk that it will lose skills that it needs, worsening the current skills shortage."

Meanwhile, the Defence Select Committee pointed out the Royal Navy had had to drop important tasks – such as counter-drugs operations and escort duties – because of its Libya commitments.

Devonport vessels played a key role. The Type 22 frigate HMS Cumberland evacuated hundreds of British and foreign nationals from Benghazi; at one point up to five Apache attack helicopters were operating from HMS Ocean; and HMS Triumph, the Trafalgar-class submarine, fired Tomahawk cruise missiles.

The warnings comes amid heightened tensions in the South Atlantic ahead of the 30-year anniversary of the Falklands conflict.

Alison Seabeck, Labour MP for Plymouth Moor View and Shadow Defence Minister, said cuts have been made "too fast and this leaves gaps in the UK's capability".

Defence Secretary Philip Hammond claimed the Libyan operation had shown the UK still had the ability to project military power around the world.

On skills shortages, he said: "We recognise the risks the NAO identifies and are actively managing them."