Britain's ageing fleet of hunter-killer nuclear submarines is suffering technical problems because of the decision to extend their service beyond their originally planned life-span, a watchdog's report has warned.

The report, released without fanfare by the Ministry of Defence, issued an "amber" warning about the Trafalgar-class subs, but stressed that an "adequate" standard of safety has been maintained to protect crews, defence workers, the public and the environment.

The Defence Nuclear Safety Regulator's report for 2012/13 also raised a "red" alert about the shortage of trained experts available to the Royal Navy to maintain and repair reactors, and warned that the situation is likely to worsen as competition for staff increases with the expected construction of a new generation of civil nuclear plants.

"The ability of the department to sustain a sufficient number of nuclear suitably competent personnel is a long-standing issue and is again raised as the principal threat to safety in the defence nuclear programme in the medium term," said the report.

"Pressure from the civil nuclear market will continue to drive vulnerability in this small and highly skilled group. Safety has not been compromised but the loss of resilience increases the likelihood of programme delays."

The report did not specify the technical problems suffered by the fleet of five Trafalgar-class subs - between 22 and 29 years old - currently operated by the Royal Navy. But with increasing age, concerns can be expected around cracks in metal or pipes and leaks of radioactive material. The oldest of the subs, HMS Tireless, suffered a leak of radioactive coolant from its reactor during operations off the west coast of Scotland in February this year.

Delays in the construction of the Astute-class subs have meant the retirement of the Trafalgars being put off, with the last one not expected to be mothballed until 2022.

The DNSR report said: "Due to build delays with the Astute class, there has been a requirement to extend the Trafalgar class beyond their original design life in order to maintain the SSN (ship submarine nuclear) flotilla at a fully operational level. Some of the emergent technical issues affecting the Trafalgar class over the last few years can be directly attributed to the effects of plant ageing."

The report said the Trafalgar class subs are "operating at the right-hand end of their 'bathtub' reliability curves and the effect has been seen in a number of emergent technical issues over the last few years". It stressed that technical issues were being managed effectively as they arise to maintain nuclear safety.

But it warned that the Vanguard class of subs which carry Britain's Trident nuclear deterrent can also be expected "to exhibit plant ageing effects as a result of the Strategic Defence and Security Review decision to extend their life to the late 2020s and early 2030s".

The Royal Navy base at Faslane on the Clyde is home to the submarines and the warheads are stored at Coulport on Loch Long when they are not at sea.

Responding to the report, a Ministry of Defence spokesman said: "We would not operate any submarine unless it was safe to do so and this report, the purpose of which is to inform best practice, acknowledges that we are taking the necessary action to effectively manage the technical issues raised by the regulator.

"It also highlights that the MoD is committed to maintaining expertise in submarine technology and operation - underlined by last month's operational handover of the first two Astute Class submarines."

An earlier version of this article implied that the Trafalgar class of submarines were based in the Clyde and carried Trident warheads. We apologise for this confusion.