Defence Secretary Philip Hammond has accused a committee of MPs of failing to recognise the need to counter evolving threats, such as potential cyber-attacks, meant there had to be a shift in the balance of defence spending.
Mr Hammond said:
It is not possible to maintain traditional regular forces at historic levels while also investing in countering the threats of tomorrow.
The Army 2020 structure is not simply about a reduction in size; it is a complete overhaul of how the Army works to deliver a fully-integrated force, using a better mix of regulars, reserves and contractors to get the maximum defence effect from the budget.
An influential group of MPs has warned that if ministers attempt to make any further cuts, the whole Army 2020 plan would "unravel" completely.
Under the plan, the size of the regular Army is being slashed from 102,000 troops to 82,000 while the numbers of part time reservists will expand to 30,000 by 2018.
In a hard-hitting report, the Commons Defence Committee said the rationale for the plan remained untested while the "high level of change" involved - at a time when troops were still engaged in Afghanistan - could "compromise" its ability to respond to emergencies.
It complained at the "apparent lack of consultation and involvement" of the head of the Army, General Sir Peter Wall, who was simply told by the senior civil servant at the Ministry of Defence what the future strength of the Army would be.
Government cuts to the Army have not been thought through and could leave Britain dangerously exposed in the event of a future crisis, MPs have warned.
The Commons Defence Committee said the restructuring progamme - known as the Army 2020 plan - was driven by the need to fit a "financial envelope" rather than a proper assessment of potential threats.
It has urged the Ministry of Defence to draw up contingency plans for a rapid recruitment programme in case there was urgent need for more troops to deal with an emergency.
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Over 1,600 servicemen and women have helped with the flood relief effort today across the UK, the Ministry of Defence said.
A statement said: "Today, military personnel continued to provide flood relief in affected parts of the UK.
"More than 1,600 servicemen and women have been committed to tasks including assisting residents, as well as filling and distributing sandbags in six different locations in the Thames Valley."
The statement added that 100 personnel carried out sandbagging and humanitarian duties in Wraysbury, 100 Royal Marines assisting people in The Somerset Levels and more than 2,000 staff on "high readiness ready to respond to short notice requests."
An ex-private says he attempted suicide several times after his complains about being bullied were ignored by the army.
Joseph McCabe is taking civil legal action against the Ministry of Defence for its alleged failure to act, the BBC reported, as well as appealing against a decision to deny him financial compensation. He said:
I'm still having nightmares, I'm still having flashbacks. If I could I would lock myself up in a box and just hide away. But if I do that it's like I'm letting those people in the Army win so I have forced myself to take up a new career, to rebuild my life.
He said that he received death threats and was stabbed in the leg at the height of constant abuse centred on his stutter but that officers laughed off the threats and no-one was punished. The Ministry of Defence responded:
Whilst we can't comment on individual cases, we can be clear that the armed forces have a zero-tolerance policy towards all forms of bullying, discrimination and abuse.
All allegations will be fully investigated either by the civil or the military police and appropriate action will be taken.
More than 1,500 military personnel will lose their jobs in the final round of armed forces redundancies, Defence Secretary Philip Hammond has confirmed.
Redundancies will include up to 1,425 members of the Army, up to 70 medical and dental officers from the RAF and up to 10 from the Royal Navy, Hammond told MPs in a Commons statement.
The fourth tranche of redundancies is part of an armed forces re-structuring programme which will see the regular Army cut from 102,000 to 82,000 by 2020, while the newly-renamed Army Reserve - formerly the Territorial Army - is being expanded from 19,000 to 30,000.
Major General David Cullen, assistant chief of general staff, said the latest tranche of redundancies "should not come as a surprise to anybody".
"It is the beginning of the end of uncertainty for our people across defence and in particular the Army," he said.