The Ministry of Defence has a funding black hole of at least £7bn in its 10-year plan to equip the UK's armed forces, according to a report by the Commons spending watchdog.
The Public Accounts Committee said the MoD lacked the ability to "accurately cost programmes" and that the shortfall could reach £14.8bn by 2028.
MPs added the government did not have a "coherent and credible" funding plan.
The MoD said it was "addressing the financial challenges" it is facing.
A spokesman for the department added: "We are confident that we will deliver the equipment plan within budget this year, as we did last year, as we strive to ensure our military have the very best ships, aircraft and vehicles."
The MoD plans to spend more than £180bn on new warships, submarines, jets and armoured vehicles over the next decade.
But the PAC's chairwoman, Labour MP Meg Hillier, said the department was "a repeat offender" when it came to "poor financial planning".
She added that progress in addressing concerns raised by the committee in May 2018 had been "woeful".
Ms Hillier told the BBC's Today programme: "There's a very big funding gap in what the Ministry of Defence says it wants to do and the money available. If they don't plug that gap there's going to be ongoing problems."
Asked if the MoD was always facing funding difficulties, Ms Hiller said: "It does feel a bit like Groundhog Day. There just needs to be a clear decision about what the priorities are.
"Any indecision or any delay has a huge knock-on effect. Small delays can cost millions, if not billions, of pounds."
Lord Dannatt, a former head of the Army, told the programme the defence secretary had "a very strong case to make" for increasing the military budget.
A defence and security review that followed the 2010 general election had "slashed and burned a lot of our capabilities", he said.
"The Americans wring their hands now and say that we're not the reliable allies that we were because we haven't got the fully rounded capabilities.
"You can't have a nation like ours' defence on the cheap. Two per cent of GDP is the lowest we've ever spent on our defence since the second world war, and it's bought us the smallest army, navy and air force we've ever had."
In its report, the PAC demanded more information on the risks associated with major projects, including the purchase of F-35 stealth jets and Type 31e frigates.
Part of the problem, the committee said, was that the government had dithered over which projects to fully finance and which to cancel or scale back.