In a new report, the NAO warned the MoD's Equipment Plan 2018 to 2028, which sets out the equipment and support budget for the next ten years, remains unaffordable.
Existing equipment, such as the Typhoon combat aircraft, and equipment in development, such as four new nuclear-armed submarines, are included in the Department's plan.
The Armed Forces' plan however, is not financially viable, the NAO argued, pointing out that the MoD expected costs of $251 billion (£193.3 bn) exceed its budget of $242 billion (£186.4 bn), which includes a $8 billion (£6.2 bn) contingency.
The NAO recommended the Ministry of Defense start saving now, as opposed to relying on long-term cuts. Poor financial planning may see Gavin Williamson's Department facing the worst-case scenario of costs increasing by £14.8 billion should all the identified risks materialize.
The NAO recommended that the Defense Ministry decided which programs to defer, de-scope or delete as soon as possible.
"The Equipment Plan 2018-28 shows that the Ministry of Defense has a clearer understanding of the affordability issues that it faces, but it equally shows how urgently it needs to get on and tackle them," head of NAO, Amyas Morse, said in the light of the new report.
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Mr. Williamson has also previously stressed to UK parliamentarians the significance of raising spending on defense, in the light of Britain exiting the European Union and aiming to forge even closer ties with its NATO partners.