The army’s bomb disposal team was called to the Merebank site where waste operator Sita is building a new recycling plant.
The site has previously been home to a former smelting works and a mustard gas factory during the First World War.
A spokeswoman for Avon and Somerset police told the Bristol Evening Post: “We were alerted to the incident at Merebank development site in Avonmouth at about 10.45am on Monday.
“Construction workers had uncovered a possible incendiary device at the scene on an old mustard gas factory.
“We called EOD (the army’s bomb disposal team) and a cordon was put in place. They attended and two devices were removed but it is believed there are a number of other devices that may be buried. These will be located and uncovered.”
Earlier this year two workers reported nosebleeds while working on the site and work was suspended while a team of experts from the Ministry of Defence were called in.
The MoD team gave the site the all-clear after finding “no evidence of mustard gas or munitions”.
A spokesperson for SITA UK said: “On the morning of Monday, July 30 contractors working at a brownfield site in Avonmouth stopped work after seeing a suspicious metal object.
“The bomb squad were called and attended site. They removed two objects from site, a spent shell and another piece of metal.
“The MoD is to provide further advice to SITA UK about further works in the specific area where these items were found. In the meantime work is now suspended in that area, which has been isolated. ”
A spokesman for the MoD said one empty shell was removed from the site and taken back to the EOD base at Tidworth in Wiltshire for disposal.