Ministers have been accused of putting Britain's military capabilities at risk by "bungling" money-saving army reforms.
The Public Accounts Committee (PAC) criticised the Ministry of Defence (MoD) for failing to consult the head of the Army fully before pushing ahead with deep cuts to regular forces.
It also gave little consideration to whether it was possible for reservist numbers to be near-doubled by 2019, and it managed to squander millions of pounds through a flawed deal with Capita.
The verdict, delivered in the cross-party committee's report on the controversial Army 2020 programme, comes as Nato leaders gather in Wales for a key summit.
The scheme is intended to save more than £10 billion over a decade, by cutting 20,000 personnel from the regular army and increasing reservist strength to 30,000.
But the National Audit Office (NAO) has suggested that the recruitment target could take six years longer than planned to hit.
The committee said it "remained to be convinced" that the goal could be reached on time.
Chair Margaret Hodge described the handling of the project so far as "astonishing".
"The decision to reduce the size of the Army was driven by the need to make financial savings in a time of austerity," she said.
"However, it is remarkable that the chief of the general staff was not involved in all stages of the decision-making process, given the magnitude and importance of the change required, and its impact on the service which he commands.
"The MoD did not test the feasibility of recruiting and training the 30,000 reserve soldiers it needs by 2019. The strength of the Army Reserve has stayed at around 19,000 for the last two years, and we remain to be convinced that the MoD will recruit the required numbers in time.
"The Army told us that shortfalls in recruitment are increasing the risk of capability gaps emerging in some parts of the Army's structure. This in turn increases the risk of additional pressure being placed on regular troops."
Capita, the private firm brought in to handle recruitment, was said to have brought in just 2,000 reserves in 2013/14 against a target of 6,000.
The goal for recruitment of regulars was also missed by 30%, according to the committee.
The company was still awarded its full bonus for recruiting reservists.
"The MoD's bungling around the recruitment contract with Capita has meant at least £70 million of the planned £267 million savings from the contract have already been lost," Mrs Hodge said.
"There was no clear understanding of the scale of the recruitment challenge, poor information about potential recruits and the MoD did not provide Capita with the IT infrastructure it needed."
Labour MP Mrs Hodge said it was "wholly unacceptable" that the Government's Plan B for the Army seemed to be "to get Plan A to work".
The committee heard that contingency plans included extending tours of duty for regular troops, which could affect morale.
The MPs also criticised the MoD for failing to provide the NAO with information it had requested on time.
"This must not happen again," Mrs Hodge said.
Shadow defence secretary Vernon Coaker said the report showed a "litany of catastrophic failures".
"These criticisms are hugely embarrassing for the Government during the week that the UK is hosting the Nato summit and when David Cameron has called for Britain's international partners to meet their defence obligations," he said.
"His advice to the neighbours would be better received and have greater credibility if he had his own house in order."
Defence Secretary Michael Fallon said: "The chief of the general staff and I are confident that we will reach our target of 30,000 trained army reservists by 2018/19. Indeed, we have arrested the many years of decline and neglect that has plagued our reserve forces and now we need to build on that.
"Our Army 2020 plans are on track and will deliver by 2020 the army we need to counter the wide range of threats we face.
"Working closely with the Army and Capita, we have already addressed many of the points raised in this report, including streamlining medical and security clearance procedures, with further planned improvements on the medical process, empowering and incentivising local units to recruit, and running a high-profile recruitment campaign.
"This is seeing a steady stream of recruits coming through the door. Our targets are challenging but they are achievable. With a Government investment of £1.8 billion in better training and equipment, I believe that many young people will realise the great offer we have for challenging and exciting roles in the reserves."