A “significant fire” has broken out at the BAE Systems nuclear submarine shipyard in Barrow-in-Furness, police have said.
Emergency services were called to the site, where the UK’s nuclear submarines are built, at about 00:44 GMT on Wednesday.
Two people have been taken to hospital after suffering suspected smoke inhalation.
Cumbria Police said there was “no nuclear risk”.
Local residents are being advised to remain inside with their doors and windows closed while the incident is ongoing. A road closure is also in place at the nearby Michaelson Bridge.
Images circulating on social media and purportedly shot at the shipyard showed large flames and thick smoke coming from a tall white building.
Police said the Devonshire Dock Hall, the site’s main building facility, had been evacuated and that everyone inside had been accounted for.
Covering around six acres, the hall is the second-largest indoor shipbuilding complex of its kind in Europe, according to BAE Systems.
The craft to have been built in the hall include the four Vanguard Class submarines that make up the UK’s Trident nuclear programme.
Four new nuclear submarines from the Dreadnought Class are currently being built at the site and are due to replace the Vanguard submarines in the early 2030s.
And the last of the Royal Navy’s seven new nuclear-powered submarines, part of the Astute Class, is also being built at the site.
BAE Systems has been contacted for comment.
Contacted for additional comment, the Ministry of Defence referred the BBC to BAE Systems while the incident was ongoing.