Dame Margaret Beckett said "a reality check is urgently required" about the challenges facing the UK (Picture: PA).
MPs and peers have warned that the new Prime Minister will need to decide whether the UK will be a significant global player and should be prepared to boost defence spending.
They believe that either Boris Johnson or Jeremy Hunt will be entering Number 10 at a time when "the cornerstones of UK national security are being undermined".
The Joint Committee on the National Security Strategy's chairwoman, former foreign secretary Dame Margaret Beckett, said "a reality check is urgently required" about the challenges facing the UK.
The committee warned that the NATO target of spending 2% of GDP on defence may not be enough for the UK "in light of both the scale and range of threats to the UK and the costs involved in keeping pace with rapid technological change".
They also called the 'Global Britain' agenda championed by Theresa May's Government "meaningless" against a backdrop of "reduced diplomatic spending and under-powered defence".
The new Prime Minister will have to "steel itself" to make difficult decisions as recently they believe that the Government, when dealing with national security, has been "talking a better game than it plays".
Dame Margaret also said: "If the Government wants to turn the 'Global Britain' concept into a meaningful strategy for a positive and self-assured role for the UK after its departure from the EU, then it needs to be more honest about how it proposes to address these challenges.
"It must also back this up with the necessary funding and resources, especially for defence and diplomacy.
"This is too important to be sidelined by Brexit.
"We call on the new Prime Minister to prioritise the UK's national security.
"Now is the time to start an honest conversation about the UK's place on the global stage, the risks we are willing to take in relation to national security and the resource which Government is willing to commit to these ends."