Troops, regulars and reserves are overstretched, the report said
Serious problems of under-manning and overstretch in the armed forces could put British troops at risk, an all-party committee of MPs has warned.
The Commons Defence Committee was reporting after a five month inquiry into the Defence White Paper.
The plans could result in troops being sent "unprepared" into "complex and dangerous situations".
But the MoD insisted work continues on plans to develop the right forces to face the new security situation.
A statement released to the BBC said the detailed plans would be revealed after Chancellor Gordon Brown publishes his Comprehensive Spending Review on 12 July.
The MPs said the MoD had prioritised hi-tech innovation at the expense of army personnel.
The MoD's plans are "depressingly short" on details of how to resolve the chronic problem of "excess stretch" in the armed forces, the report continues.
"We believe the manpower shortages must be tackled urgently," it added.
The report is also critical of plans to reduce the scale of the armed forces - especially before new hi-tech capabilities and equipment are introduced.
Under the plans, unveiled by Defence Secretary Geoff Hoon last December, the UK will reduce the size of heavy armoured forces in favour of units that can be sent into action quickly.
Committee chairman Bruce George said: "Troops, both regulars and reserves, are already over-stretched.
He told BBC Radio 4's World at One programme that British armed forces had been acting at the limits of what is sustainable for the last six years.
"Cutting the number of boots on the ground, ships or aircraft are not sensible options."
This was particularly true when there was "no guarantee" that the equipment meant to replace them would arrive "on time or perform as planned", he added.
Members said it was unclear whether the government proposals - which could lead to huge changes in the armed forces - "look forward" genuinely or whether they have been "resource driven".
MPs are also critical of the White Paper's focus on fighting terrorism "at a distance" with expeditionary forces.
Mr George says cutting troop numbers is not a sensible option
Government departments have thus paid insufficient attention to the military's role of defending the UK itself against terrorism, they said.
Mr George continued: "If terrorists decide not to play by our rules, then we will need forces that can react quickly to threats abroad and at home."
This will only be possible, he says, if troops are fully trained to deal with the new demands.
"We are not convinced that the MoD is on top of this problem," he added.
He called for a more imaginative approach to the role of the military in the event of a terror attack in the UK.
The Home Office, MoD and other government departments should work more closely together on this issue, he added.
"Their discussions need to be about defending the homeland not defending departmental turf," Mr George said.
Tory defence spokesman Keith Simpson said his party had been urging the government to appoint a homeland security minister for some time.
"The real task of dealing with any terrorist attack and its aftermath would be in the hands of some specialist TA companies that have been set up," he said
As things stood, these were understaffed and undermanned, he added.
Liberal Democrat defence spokesman Paul Keetch described the report's criticism's as "stinging" and "unprecedented".
"Instead of increasing numbers to relieve over-stretch, the MoD has been encouraging people who've signed off to leave early," he said.
"Urgent action is needed to get all the services back up to strength."