Plans to buy new military equipment - including warships and jets - could be under threat, as MPs expressed "serious doubts" over whether the Ministry of Defence can afford them.
The Commons Defence Select Committee says the department will struggle to find the £7.3bn in savings required to pay for the new hardware.
It said the MoD had proved "incapable" of making such savings in the past.
The MoD said it was making "good progress" on its efficiency target.
This latest report echoes a warning from January of this year from government spending watchdog, the National Audit Office.
The cross-party committee of MPs backed the findings of the NAO that the defence equipment plan was at "greater risk" than at any time since 2012.
In 2016, the government pledged to spend £178bn on new military equipment over the next 10 years.
But that is on the assumption it can also find £7.3bn of efficiency savings - on top of £7.1bn previously announced - by selling off property and other efficiencies.
The committee chairman, Conservative MP Julian Lewis, said this was now "extremely doubtful" from an "already stretched budget".
"This will inevitably lead either to a reduction in the numbers of ships, aircraft and vehicles or to even greater delays in their acquisition," he added.
But the MoD said that in the face of "intensifying threats", its £178bn equipment plan "continues to deliver the cutting-edge kit to keep the UK safe".
The new equipment covered by the plan includes eight Type 26 frigates for the Royal Navy, new mechanised infantry vehicles and nine Boeing P-8A maritime patrol aircraft.
The committee also called for clarity on the "difference between genuine improvements and efficiency" and cuts to "personnel, equipment and capability".
Defence Secretary Gavin Williamson has already been warned of a Tory revolt over cuts to army numbers and naval capability, and suggested he will be asking the chancellor for more money.
The Cabinet Office is currently carrying out a defence and security review which is due to report by the end of the year.