A decision to cut regular armed forces, while doubling reservists, was "taken without appropriate testing of feasibility" the National Audit Office have said.
The National Audit Office issued a statement saying that "military judgment played an important role in decisions."
They added: "Committing to moving towards an Army structure with fewer regular soldiers and an increased number of reserves within the planned timescale, should have been subject to more rigorous testing of feasibility."
Regular forces numbers will drop from 102,000 to 82,000 by 2018, with the report saying that without a "significant change in performance", the target may not be hit until 2025.
An annual shortfall in army savings will need to be found from cuts across the armed forces "other than equipment," the National Audit Office said today.
ITV News' Deputy Political Editor Chris Ship tweeted:
The £1.1bn annual shortfall in savings will need to be found from cuts "across the armed forces from budget areas other than equipment" <NAO
Despite cutting regular Army by fifth to save cash @naoorguk estimates MoD won't make savings it had planned. Will be £1.1bn PER YEAR short
NatAuditOffice: "significant risks" in new Army structure & shortfall in recruitment of new reserves "will up the pressure on regular units"
Cuts to the size of the Army pose "significant risks" which could "significantly affect the Army's ability to achieve its objectives and value for money", the National Audit Office said today.
The first three female submariners to serve in the Royal Navy said they were looking forward to getting to work with their male colleagues and expect to be treated professionally.
Lieutenants Maxine Stiles, Alexandra Olsson and Penny Thackray completed vigorous training that enables them operate the submarines, which they explained are "more complicated than a spaceship". Lieutenant Alexandra Olsson said:
"I am sure it will be a big success. We all do our job, and as long as you can do your job on board, everyone is quite happy for you to be there".
Three officers have made maritime history by becoming the first female submariners to serve in the Royal Navy.
Lieutenants Maxine Stiles, Alexandra Olsson and Penny Thackray have completed months of specialised training to earn their "Dolphins" - the clasp worn by qualified submariners - becoming the first women in the 110-year history of the Navy's Submarine Service.
For years women were unable to serve on submarines because of possible health risks but, after an independent review found that only pregnant women should not serve, Defence Secretary Philip Hammond lifted the ban in December 2011.
Following the arrival of woman officers, female ratings (non-commissioned personnel) will start training later this year with a view to serving on Vanguard submarines in 2015. Female personnel will also be able to serve on Astute-class submarines from around 2016.
Britain's ability to mount military operations risks being undermined by the "unprecedented" number of legal challenges being brought against the Ministry of Defence and the armed forces, MPs have warned.
The Commons Defence Committee said the fear of legal action meant troops on the battlefield may be increasingly unwilling to assume responsibility or take the necessary risks - damaging operational effectiveness.
It said the "judicialisation" of warfare could even lead to bloodier battles and more civilian casualties with risk-adverse commanders increasingly opting to call in airstrikes rather than put their troops in the line of fire.
The Malaysian transport minister's communications team has posted an image of him talking on the phone - in his dressing gown - to Defence Secretary Philip Hammond about the Royal Navy submarine arriving to help search for the missing Malaysia Airlines flight:
The Ministry of Defence has defended its involvement in a military training exercise that involves the shooting of live animals. Images published in the Daily Mirror appear to show live pigs strung up as targets during a medical training course at Nato's training facilities in Jaegerspris, Denmark.
The pigs are shot by a marksman to replicate battlefield wounds so that military medical staff can train in emergency surgery. The MoD said it had not yet seen the pictures, but a spokeswoman said it was normal for military surgeons to carry out training on animals:
"Our military surgeons undertake vital training in Denmark where they learn specialist trauma treatment skills that save lives on the battlefield.
"All animals used in medical training are anaesthetised before they are treated and by participating in the Danish led exercises twice a year rather than conducting our own, we minimise the overall number of animals used."
Animal rights group People for the Ethical Treatment of Animals (Peta) said the exercise was "impossible to justify medically, ethically and educationally."
The crew of a Royal Navy destroyer have sent a Valentine's message to their loved ones back home from the warship's flight deck.Read the full story ›
A military dog belonging to British armed forces has been captured by militants in Afghanistan, ITV News understands.
The Ministry of Defence has refused to comment on reports that the animal, which has appeared in videos on jihadi websites surrounded by armed men, is British.
Media reports claim the dog is called Colonel and is believed to have been captured at the end of last year, nothing is known about his current whereabouts.
The International Security Assistance Force (ISAF) has confirmed a military dog in Afghanistan went missing in December.