A former Chief of the Defence Staff has warned that reservists must not be used as a "cheap and cheerful" alternative to the regular forces.
Independent crossbencher Lord Jock Stirrup said the armed forces reserves had an important part to play in the mix of military forces to defend the UK.
“[Reserves] make a unique contribution in their own right and are a crucial part of the force structure, not merely an optional add-on.”
Latest figures show there are 32,330 reserves in the Armed Forces. The number of people joining the reserves decreased by 16 per cent in the last year.
Lord Stirrup stressed that reserves had to be adequately funded and not used as a "fig-leaf" to cut costs, amid fears about the possible impact of the proposed hike in NHS spending on defence and other budgets.
He said the size of the "reserve component" should be considered in its own right, "not as a way of providing some sort of cheap and cheerful alternative to regulars".
Reservists were not regulars and could not be used for roles that required regular service but were "important in their own right".
Lord Stirrup told the Lords: "If reservists are not properly trained and equipped if the Government uses them merely as a fig-leaf for force structure reductions in order to cut costs, they will not stay.
"The regular reserve mix should be determined by overall capability, not by financial expediency."
In a debate on the contribution made by the reserves to national security, Lord Stirrup said the future of the armed forces was "in the balance" due to the Modernising Defence Review.
He said: "It's no secret that in order to deliver the force structure set out in the conclusion of the 2015 defence review the Ministry of Defence will require additional funds.
"But it is by no means clear that these will be forthcoming."
He warned ministers that "many of us are watching carefully to see if this Government intends to discharge properly what is acknowledged to be its first duty to our citizens, their defence and security.
"If it should fail to do so, it should not expect us to sit idly by," he added.
For the Government, Baroness Goldie paid tribute to the "tremendous contribution" made by the reserve forces and insisted investment in them continued to be strong.
She said the UK was one of "very few" allies to meet the NATO spending guideline of two per cent of GDP on defence.
The budget would rise by 0.5 per cent above inflation every year of this Parliament.
Lady Goldie dismissed reports that the Prime Minister had cast doubt on the UK's status as a "tier one" military power, insisting:
"The Government is categorically committed to retaining the UK's position as a tier one defence nation."