THE Royal Navy's hunter-killer nuclear submarines are increasingly reliant on parts from other vessels to stay operational due to savage cuts in defence maintenance budgets, spending watchdogs have warned.

Research shows that cannibalisation of parts across the Navy has increased by 49per cent over the past five years with the multi-billion pound Clyde-based Astute class of submarines relying on spares the most.

According to the National Audit Office there were 795 cannibalisation incidents, which is otherwise known as “stores robbery” across the fleet last year.

Official Ministry of Defence guidelines does not outlaw the practice but states it should only be used “when no other solution is is available”.

But the NAO discovered is it is being used 66 times a month this year compared to just 30 in 2005.

It comes as the Royal Navy maintenance budget is slashed by £62 million a year meaning engineers do not have spare parts available.

Even parts from submarines under construction at a cost of £1.6 billion each are having parts removed to keep the fleet operational before they a have left the shed.

According to the spending watchdog, the new Clyde-built Type 45 destroyers are also relying on spare parts from other vessels just to keep afloat.

Of the cannibalised parts, 22 were priced at less than £1, 192 were less than £10 while the vast majority - 991 - were prices between £10 and £99.99.

In total 71 per cent of all cases cost less than £500 while on 14 occasions parts worth £1million or more were swapped.

The National Audit Office report said: “Equipment cannibalisation can be necessary but should only happen when no other solution is available. “In the last five years, between 0.3per cent and 1.4per cent of parts provided to the main classes of ships and submarines have been cannibalised parts.

“However, each instance has a wider impact beyond the part being replaced and can signify broader issues with the process for getting spare parts. “ “The Ministry has taken decisions to reduce support without complete information to fully assess and manage the impacts and costs.

“To remain within its budget, the Navy reduced its maritime support budget by 6per cent (£271 million) in-year in the last two years.

“Of these reductions, an estimated £92 million could increase the need to cannibalise parts.

“Each instance of equipment cannibalisation can delay programmes, create additional engineering risks and add to the work of staff, affecting morale.

“Cannibalised parts, along with additional parts that must be removed to gain access to them, may be damaged whilst being removed, transported or re-installed.

“An estimated 11per cent of the parts recorded by ships as having shortcomings in their material, design or documentation were cannibalised.

“The 2017 Navy risk register identified a lack of spare parts as a risk to operational capability given its demoralising effect on personnel.”

The list of equipment includes weapons systems, sonar and radar and submarine escape equipment but the most common faults are with “non-vital” valves.

The MoD claims it cannot hold spare parts for every contingency as it would not be good value for money, but it said it would monitor the longer-term impact of relying on cannibalisation.

The multi-billion-pound hunter-killer submarine, based at Faslane, has already faced a string of problems and has been unable to reach its top speed, had electrical problems and has suffered corrosion.

Last year the entire fleet of Type 45 destroyers, the Royal Navy’s most powerful warships are in port at the same time but apparently not on operations.

The destroyers have experienced mechanical problems in the Persian Gulf with engines breaking down when the water becomes too warm.

A Royal Navy spokesman said: “Less than half a percent of parts we use come from swapping components, and we only do this when it’s absolutely necessary to get ships out of port and back onto operations more quickly. We continue to make improvements to how we manage this long-established practice.”