Staff at the BAE Systems facility at Barrrow have been evacuated after reports of a bomb onboard an Astute class submarine.
Emergency services were called to the Devonshire Dock complex at 9:17am.
A BAE Systems spokesman said: “We can confirm there is an ongoing incident at our Barrow site and we are liaising with Cumbria Police who are carrying out an investigation.
“As a precaution the Devonshire Dock complex has been closed.
“Staff, contractors and local residents are being kept informed.
“There is no perceived threat to the wider public at this time and the rest of the site is in operation as usual.”
It’s believed that 1,700 staff were evacuated.
A Cumbria Police spokesman said: “Police are on-site at BAE assisting staff.”
A North West Ambulance Service spokesman said: “We are currently on scene but there are no known casualties.”
A BAE spokesman added: Following an extensive sweep of the Devonshire Dock Complex (DDC), including the four Astute class submarines in build, nothing suspicious was found. The situation is being managed in accordance with normal procedures and we expect to be able to close the incident shortly.
“Our first priority is always the safety of employees, the site and local residents so in accordance with our emergency procedures, we made the decision to release personnel from the DDC earlier today. This is a well-exercised procedure and was purely precautionary.
“Working with the emergency services, we are now satisfied there is no immediate safety risk.
“Work is now returning to normal following this morning’s incident and employees on today’s back shift and night shift, as well as tomorrow’s day shift, are asked to report to work at their usual times and places.
“Further information is available via our employee hotline on: 0808 126 1908.”