document: The Office for Nuclear Regulation (ONR) has also warned Devonport in Plymouth might need enhanced monitoring until 2020 unless improvements are made.
  Special measures, including monthly inspections, were predicted to last two years when first imposed in 2013.
  Babcock, which carries out the refitting for the Ministry of Defence, said safety was its "highest priority".
  The ONR said its "enhanced regulatory attention" meant the once-a-month inspections of HMNB Devonport had "more effort and managerial attention".
  Its latest annual report blames safety concerns on "ageing facilities" and "increasing demands" as well as "stretched resources" for an unscheduled refuelling of HMS Vanguard later this year.
  Last December, Babcock was warned of legal action after a worker received a dose of radiation amid a series of safety breaches.
  ONR - KEY REPORT POINTS ON DEVONPORT DOCKYARD
  When Devonport was first placed under special measures in 2013, the ONR said the base was "expected to receive enhanced regulatory attention for around two years, as we anticipate the issues to be resolved during that time".
  It added with "sustained commitment over the next three to five years", Devonport "has the potential to move to a routine attention position".
  Peter Burt, of the Nuclear Information Service, said: "The nuclear safety improvements required at Devonport have not been completed within the expected two year timescale, and we will apparently have to wait for several more years before the situation improves.
  "Babcock is clearly unable to deliver the improvements it is being paid to complete. It is time for the MoD to cancel the current management contract for the dockyard and bring the site under the control of a new, competent management team."
  Source: MoD
  Babcock said in a statement: "The continued safety of our operations at Devonport remains the highest priority.
  "We have developed a comprehensive long term safety improvement programme to address the points raised by ONR. That work is progressing to plan and has the full visibility of the regulators."
  The Ministry of Defence said: "Safety at HM Naval Base Devonport, as with all Ministry of Defence sites, is of paramount importance.
  "The MoD supports Babcock's commitment to address the issues raised in the report, and we continue to work together to deliver the necessary investments at Devonport, which includes a site-wide nuclear safety programme with sustained commitment over the coming years."
summary: The UK's nuclear submarine refit base is to remain in special measures amid safety concerns.