Defence saving plans raise "serious doubts" over the Ministry of Defence's ability to pay for new equipment, including warships and planes.
The report warned that the MoD's already "stretched budget" left little room for manoeuvre to make a further £7 billion in savings £7.1 billion previously announced.
The MPs on the Commons Defence Committee backed the findings of spending watchdog the National Audit Office that the defence equipment plan was at "greater risk" than at any time since 2012.
The £178 billion plan set out in 2016 involves a 10 year spending plan on equipment including eight Type 26 frigates for the Royal Navy, new mechanised infantry vehicles and nine Boeing P-8A maritime patrol aircraft, but rests on the MoD making savings of £14 billion.
"Even if all the efficiencies are realised, there will be little room for manoeuvre, in the absence of sufficient financial 'headroom' and contingency funding," the MPs warned.
"This is not an adequate basis for delivering major projects at the heart of the UK's defence capability."
The committee's Tory chairman, Julian Lewis, said: "It is extremely doubtful that the MoD can generate even more efficiencies from within its already stretched budget on the scale required to deliver its equipment plan.
"This will inevitably lead either to a reduction in the numbers of ships, aircraft and vehicles or to even greater delays in their acquisition."
The committee's report is published against a backdrop of Tory unease about the prospect of cuts to military spending as part of a security capability review being carried out by National Security Adviser Mark Sedwill.
A Ministry of Defence spokesman said: "In the face of intensifying threats our £178 billion equipment plan continues to deliver the cutting-edge kit to keep the UK safe.
"As we told the Defence Committee, we are making good progress towards our efficiency target.
"We always look to provide the best value for money for the taxpayer, with all savings reinvested in defence."