The Ministry of Defence (MoD) is struggling to recruit enough skilled engineers to keep its nuclear bombs and submarines safe, according to a new report from its internal watchdog.

The Defence Nuclear Safety Regulator (DNSR) is warning that the shortage of suitably qualified nuclear staff is "the principal threat to the maintenance of safety in the Defence Nuclear Programme". This is "a long standing issue" that could impede major developments, it says.

The warnings come in DNSR's latest annual report covering 2013-14, which was posted online without announcement. The staff shortages have been caused by competition from the nuclear power industry, which the UK government is trying to greatly expand.

"Pressure from the civil nuclear market will continue to drive vulnerability in this small and highly skilled group," the report says. The sustainability of the necessary skill set within the MoD "remains fragile".

Interventions to try and tackle the shortages had so far shown "little direct evidence of improvement", DNSR says. "Safety has not been compromised, but the loss of resilience increases the likelihood of project delays."

The report also highlights threats to safety from major organisational changes within the MoD's nuclear programme. It stresses that managers had to make sure that capabilities remained robust and that "any changes to organisational arrangements are assessed for their impact on safety prior to implementation."

Dealing with staff shortages and organisational changes would require "sustained attention to ensure continued safe delivery of the Defence Nuclear Programme over the medium to long term," the DNSR report cautions.

It also lists six other strategic safety issues for the MoD. They are ageing nuclear facilities, safety management arrangements, product quality, transport package approval, nuclear liabilities and addressing issues raised by the 2011 Fukushima disaster in Japan.

The report does not explain exactly what these issues involve, but says that improvements are expected to be delivered "over a number of years". It adds: "DNSR will continue to undertake targeted inspections and audits to confirm appropriate action is being taken and to monitor progress."

According to a former senior MoD radiation safety official, Fred Dawson, the MoD had repeatedly failed to address the problem of staff shortages. "The MoD's internal nuclear safety regulator has failed to take any enforcement action against MoD management that would make take the MoD take this issue seriously," he said.

"The MoD's failure to address the lack of suitably qualified and experienced personnel makes the case for full civil regulation of the MoD's nuclear programme."

The SNP's Westminster leader and defence spokesman, Angus Robertson MP, said: "The MoD has known for years that one of the biggest safety concerns was the lack of suitably qualified nuclear safety posts - yet they have done very little to fill them."

He added: "The MoD simply cannot commit to spending an eye watering £100 billion on Trident for it to emerge that they don't have enough money to retain skilled and experienced personnel to maintain safety. It is totally unacceptable."

The Sunday Herald reported MoD figures last year showing that one in 10 nuclear safety posts were vacant. The MoD was missing 165 suitably qualified and experienced nuclear personnel on 31 March 2014, compared to shortfalls of 133 in March 2013 and 153 in March 2012.

The MoD said the government was working hard to tackle the skills gap. "Last year it announced the intention to open a new elite college offering skills training for the nuclear power sector, alongside £8 million of funding to train future nuclear engineers," said an MoD spokeswoman.

"In addition, the MoD and the Royal Navy has a range of initiatives in place to ensure we have the right skills and personnel in place to operate our submarines entirely safely." No submarine goes to sea without the minimum complement of suitably qualified and experienced personnel required to operate the boat safely.

According to the MoD, the requirement for safety-critical personnel changes frequently and vacancies occur. Vacancies are managed to ensure that safety and operational capability is never compromised, it says.

But Peter Burt, from the Nuclear Information Service in Reading, warned that sooner or later a major problem would develop. "For several years now the MoD's nuclear programme has been teetering along with the bare minimum of suitably qualified staff in post necessary to keep things safe," he said.

"The DNSR annual report admits that there is little prospect for improvement over the foreseeable future. The longer this state of affairs drags on, the greater is the risk that something will eventually collapse under the strain."