The A15 Sleaford Road is shut as bomb disposal squads deal with an unexploded device at RAF Waddington - the second to be found at the site in a week.
Wing Commander Matthew Nicholas, officer commanding operations wing at RAF Waddington said: “During excavations to the RAF Waddington runway contractors have unearthed what appears to be a second historical piece of ordnance.
“As a precaution a cordon is now in place until Military Explosive Ordnance Disposal experts who have been called in to assess the situation can dispose of the device appropriately.
“The cordon will regretfully cause a stretch of the A15 to close until the device has been disposed of.”
He added: “We apologise for any inconvenience caused but we will without hesitation; always take the necessary precautions to ensure the safety of station personnel and members of the general public.”