Failure to deliver the UK's aircraft carrier and jet programme on budget will put other defence projects at risk, ministers have been warned.
A Public Accounts Committee report said defence budgets are "very strained".
The programme - two aircraft carriers, F-35 Lightning II jets and a new radar system - leaves the Ministry of Defence "financially exposed," MPs said.
The MoD said that it was committed to keeping costs down.
It said in a statement: "This is a crucial investment that will revolutionise our ability to defend our nation as we face intensifying threats."
The approved cost for both carriers is £6.212bn, according to the MoD, which added that the carriers would have an expected service life of up to 50 years and be used for both humanitarian relief and war-fighting, representing "tremendous" value for money.
But it's not just the cost of the two aircraft carriers that concerns MPs, said the BBC's defence correspondent Jonathan Beale, but the new jets that will fly off them and the other warships needed to protect them.
The availability of ships to protect the carriers may also limit how they can be used and there are still technical and cost challenges for the new F-35 jets, he added.
The committee's chairwoman Meg Hillier said keeping the project, known as Carrier Strike, on budget will be "no easy task" due to "uncertainty over some costs and the potentially negative impact of foreign exchange rates".
"We will be keeping a close eye on this programme and will expect the Department to keep us abreast of developments," she added.
The fluctuating value of the pound has affected the cost of the jets as they are being bought from US-based Lockheed Martin. While the pound has rallied, its value remains lower than before the Brexit vote in 2016.
Labour's shadow defence secretary, Nia Giffith, said the government must "urgently address the issue and "ensure that the Carrier programme is adequately funded going forward".
The committee's warning adds to existing pressure on the MoD from former ministers and military leaders, as well as Conservative backbenchers, not to make other cuts to armed forces budgets.
The committee also said the government must ensure the new jets and carriers can be upgraded with future technology, so they are "future proof" and "value for money".
The government unveiled the first of the new class of aircraft carrier, the £3.1bn HMS Queen Elizabeth, in 2017. It later reported leaks during its first sea trial.